Human impacts on fluvial systems in the Mediterranean region

Human impacts on fluvial systems in the Mediterranean region

Geomorphology 79 (2006) 311 – 335 Human impacts on fluvial systems in the Mediterranean region J.M. Hooke Department...

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Geomorphology 79 (2006) 311 – 335

Human impacts on fluvial systems in the Mediterranean region J.M. Hooke Department of Geography, University of Portsmouth, Buckingham Building, Lion Terrace, Portsmouth, PO1 3HE UK, UK Received 26 August 2005; received in revised form 6 June 2006; accepted 6 June 2006 Available online 1 September 2006

Abstract The long history of substantial human impacts on the landscape of the Mediterranean region, and their effects on fluvial systems, is documented. These effects have included impacts of deforestation and other land use changes, agricultural terracing on a wide scale, water transfers, and irrigation schemes. During the 20th century, major changes were made directly to channels through channelisation, construction of dams of various sizes, and extraction of gravel, and indirectly by reforestation. These changes have caused a major phase of incision on some rivers. Runoff and soil erosion have been affected by types of crops and agricultural practices as well as by the varying extent of cultivation and grazing. Some recent agricultural practices involve wholescale relandscaping of the topography and alteration of surface properties of material. The importance of analysing the connectivity within different land units and of the spatial position of human activity within a catchment is illustrated. The analysis of connectivity is the key to understanding the variability of impact and the extent of propagation of effects. © 2006 Elsevier B.V. All rights reserved. Keywords: Soil erosion; Land degradation; Mediterrean; Rivers; Connectivity; Land management; River management

1. Introduction The Mediterranean region of Europe has a long history of human settlement and human impacts. Much debate has focused on the environmental effects of human land use and its relative importance compared with climatic impacts, included in the volume by Thomas (1956). This debate continues in relation to recent phases of activity and phases in antiquity. The long history of human influence in the Mediterranean region means that distinguishing human impacts poses particular difficulty. The very high spatial and temporal variability of fluvial processes in the region also creates problems for measurement and monitoring and for assessment of effects. The existence of Mediterra-

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nean climatic zones elsewhere in the world with rather different histories may provide some opportunity for evaluation. Indeed, Sauer (1956) suggested this but systematic comparison is limited as yet (Roberts, 1989; Brierley et al., 2005). After the long history dominated by the clearance of vegetation and increasing exploitation of the land, abandonment of land from agriculture in southern Europe is also providing some opportunity to compare fallow land with cultivated areas. It cannot be assumed that this represents a return to a ‘natural’ state since the long history of cultivation and land use may have completely altered the soil condition. Dedkov and Mozzherin (1992) assembled data on rates of erosion across the world and classified the degree of disturbance of catchments. They concluded that Mediterranean mountain streams exhibit the highest anthropogenic contribution of any climatic zone.


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Several reviews of physical and environmental aspects of the Mediterranean region have been published in recent years, including a general overview by King et al. (1997) and a detailed analysis by Wainwright and Thornes (2004) that focuses on environmental issues. The issue of desertification and its threat to the region has provided impetus to much research. Major projects such as MEDALUS, funded by the EU, have promoted key publications, many giving much background on the physical state and processes in the region (e.g. Brandt and Thornes, 1996; Mairota et al., 1998; Balbanis et al., 1999; Geeson et al., 2002). A recent book arising from the EU funded MODMED project (Mazzoleni et al., 2004a), concerned with changes and dynamics of vegetation, however, challenges some of the recent assumptions and views about desertification in Europe. Much data and information on Mediterranean type regions around the world and the nature, extent and causes of land degradation in each of them were brought together by Conacher and Sala (1998). In terms of fluvial systems, the early, classic work on the region was the book by Vita-Finzi (1969). Lewin et al. (1995) have more recently discussed fluvial developments during the Quaternary, including climatic and human impacts. The volume edited by Bull and Kirkby (2002), while entitled Drylands Rivers, has a major focus on the Mediterranean region and extends from runoff processes to channels. A review by Poesen and Hooke (1997) focuses on present understanding of erosion, flooding and channel management in the European Mediterranean region and identifies research gaps. This paper reviews the impacts of human activities on fluvial systems in the Mediterranean region, focusing on changes occurring in the 20th century and currently. Against the background of the present characteristics and dynamics and the longer-term history, it identifies the main activities and discusses the nature of their influences. It examines the functioning of the system as a whole and considers the implications of the current scales of activity. The discussion is set within the framework of connectivity (Hooke, 2003) of the system and the changes which human activities have made to this, so influencing the delivery of water and sediment down through the system from source areas to the coast. The major human impacts over the last century or so tend to have been related to water and channel management, to land use changes and land practices, and more recently to urbanisation. Some case studies are taken in particular from southeastern Spain. This is the driest part of the region and arguably can show trends which may occur elsewhere under scenarios of global warming, and also very rapid and large scale transfor-

mation of landscape and land use is currently taking place there. Finally, the region has been the focus of much research. 2. Characteristics of the Mediterranean region Definition of the Mediterranean is quite difficult and even in major publications on the region, definitions are avoided or various alternatives are proposed (e.g. King et al., 1997; Wainwright and Thornes, 2004). In Koppen's definition it is the climatic zone in which winter rainfall is at least three times that of summer (Palutikof et al., 1996). The region is often considered as that bound by the limits of growth of olive trees. Conacher and Sala (1998) agree that the most distinctive characteristic is summer drought. The high seasonality of the climate, with mild, wet winters and hot dry summers, leads to particular characteristics in the hydrological regime. Rainfall varies within the region, ranging from semi-arid, <300 mm in SE Spain, to >1200 mm in parts of Italy and France, and higher still in parts of the Balkans. The peak rainfall varies slightly in its timing across the region but is mostly in the transitional months of autumn and spring. The climatic and hydrological characteristics are also complicated by the presence of high relief in much of the region, producing sharp climatic gradients away from the coast and altitudinally. As a result of these climatic and physical characteristics the region experiences high spatial and temporal variability of rainfall. The runoff regime has generally been characterised by the dominance of Hortonian overland flow but, even in the driest parts of the region, this may be an over simplification (Scoging, 1989; Beven, 2002). Saturation excess overland flow may also occur; for example, Calvo-Cases et al. (2003) identified both as occurring on limestone slopes in SE Spain. Subsurface seepage within regolith layers may also contribute under certain conditions and piping is an important process in some materials and locations. Beven (2002) emphasises the spatial and temporal variability of runoff even within a storm and the case study by Bull et al. (1999) shows the importance of intense pulses of rain even within a prolonged period of rainfall. Much recent work has demonstrated the importance of the distribution of vegetation and its interaction with runoff at the patch scale, with runoff source (bare) and sink (vegetated) areas (e.g. Puigdefabregas et al., 1998, 1999). Severe problems of heterogeneity, however, emerge in scaling up from plot and hillslope to catchment scale, as demonstrated by Cammeraat (2004). Much current research on modelling runoff is using concepts and

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identification of homogenous hydrological, morphological or geomorphologic units within hillslopes (e.g. Bracken and Kirkby, 2005). Even storms in which the rainfall threshold for runoff generation is exceeded often do not lead to sufficient runoff to reach main channels and for flow to be generated down through the catchment. River channel flow in the Mediterranean region varies from perennial to ephemeral, mainly in relation to overall rainfall and humidity, but even within one area (VidalAbarca et al., 1992) a range of flow regimes can be found, mainly controlled by lithology. Spatial variability in hydrological response and the major influence of lithology is illustrated by records of stream flow collected between 1997 and 2005 in three ephemeral channel systems within the Guadalentin basin in SE Spain (Fig. 1). The Nogalte is in schist and is much less responsive than the other two catchments. Torrealvilla is on Quaternary gravels and marl, and the most responsive is the Salada catchment, which is in marls. Likewise, for the same area Bracken and Kirkby (2005) show the differing responsiveness and erosion on schist and marl within one storm and Shannon (2002) discusses the high transmission losses in channels. Erosion rates and sediment yields from hillslopes are significantly related to lithological and soil variations. MartinezMena et al. (1998, 2002) demonstrated how the high spatial variability of runoff and erosion is controlled by the intensity of rainfall and surface properties. Low rates of erosion occur from limestone though it forms some of the highest mountains in the region (Woodward, 1995). Arguably, the key factor and issue in understanding the Mediterranean environment is that of vegetation. Much of the semi-natural vegetation in the region is now characterised by two types of shrubland, maquis and garrigue, with woodland, especially evergreen oaks and pine species, in uplands. Much debate occurs about the original cover and the degree of human transformation, particularly whether all parts of the region were at one time forested and whether the Mediterranean scrub and heathlands are only maintained by cultural factors, such as grazing by sheep and goats. (e.g. Rackham, 1982; Roberts, 1989; Rackham and Moody, 1996; Grove and Rackham, 2001). Recent work (e.g. Lawson et al., 2005) suggests that woodland was present, for example, in northern Greece in the mid-Holocene and then impacts of human influence become apparent. Vegetation influences runoff and erosion through interception, litter cover and root density, and effects on infiltration and crusting. Thornes (1990) has shown that a threshold for


erosion occurs at about 30% cover. Some research has been done into rates and effects of colonisation and spread of vegetation, particularly on badlands and on abandoned agricultural land (e.g. Alexander et al., 1994; Gallart et al., 2002; Obando, 2002). The high relief bordering much of the Mediterranean Sea means that the region is characterised by short steep river systems. Macklin et al. (1995) have used the position of the 500 m contour to show the upland nature of most catchments in the Mediterranean basin. Poulos and Collins (2002) indicate that small rivers, classified as < 10,000 km 2 , drain about 50% of the total Mediterranean Sea catchments. The morphology of river channels ranges from confined narrow channels in canyons, flowing within upland areas, to wide, braided channels in piedmont areas and large valleys. The channels vary markedly with the type of sediment supply (Hooke and Mant, 2002b). Many of the confined channels represent relatively rapid long-term incision, because of active tectonics in much of the region. Flashy flow regime and marginal growth of vegetation mean channels tend to be unstable and variable in morphology over time, (Graf, 1988; Conesa-Garcia, 1995; Hooke and Mant, 2002b). The climatic regime and the juxtaposition of highland produce a propensity for extreme flooding in many areas. The lack of long records from river gauges and the sporadic nature of flooding cause difficulty in assessing changes in flood frequency, though the depth of historical record does help. Poesen and Hooke (1997) review the occurrence of extreme events in the region and Lopez-Bermudez et al. (2002) appraise the occurrence of floods in ephemeral streams of Mediterranean Spain. Their data indicate an increased frequency of floods over the past eight centuries, with particular increase in the 18th and 19th centuries, which they attribute primarily to deforestation. A very large increase also occurs in the 20th century. Debate on the relative influence of human activities and of climatic fluctuations continues in current work on palaeohydrology and flood frequencies (e.g. Benito et al., 1998; LopezBermudez et al., 2002; Maas and Macklin, 2002; Thorndycraft et al., 2002). High intensity, localised storms can lead to major flood disasters, such as that at Puerto Lumbreras, in SE Spain in 1973 in which 250 mm of rainfall fell in 6–8 h and produced an estimated flood peak of 1160 m3 s− 1 (Thornes, 1998; Lopez-Bermudez et al., 2002). The high spatial and temporal variability of floods is exemplified by Camarasa and Segura (2001) in Valencia, Spain. Sediment loads tend to be much higher in ephemeral streams than in perennially flowing streams (Reid and

314 J.M. Hooke / Geomorphology 79 (2006) 311–335

Fig. 1. Frequency of stage heights measured at three monitoring sites in each of three catchments in the Guadalentin basin, Murcia, Spain in the period 1997–2004.

J.M. Hooke / Geomorphology 79 (2006) 311–335 Table 1 Sediment yields at basin scale in the Mediterranean region River


Catchment Sediment Reference area 102 transport/yield (km2) t km− 2 a− 1

(a) Based on sediment transport measurements at catchment scale (from Wainwright and Thornes, 2004) Segura Spain 230 LópezBermúdez (1979) Segura Spain 3000 Romera Diaz et al. (1988) Tordera Spain 20 Sala (1982) Bradano Italy 1159 Rendell (1986) Sinni Italy 2458 Rendell (1986) Crati Italy 1003 Rendell (1986) Tiber Italy 377 Rendell (1986) Arno Italy 250 Rendell (1986) Yael Israel 390 Schick (1977) Hillazon Israel 158 23 Inbar (1992) Netofa Israel 121 190 Inbar (1992) Qishon Israel 470 180 Inbar (1992) Qishon Israel 224 50 Inbar (1992) Snunit Israel 65 45 Inbar (1992) Alexander Israel 544 16 Inbar (1992) Ayalon Israel 160 117 Inbar (1992) Eqron Israel 62 185 Inbar (1992) Soreq Israel 80 47 Inbar (1992) Pelugot Israel 200 200 Inbar (1992) Shiqma Israel 746 160 Inbar (1992) Adorayim Israel 86 165 Inbar (1992) Lahav Israel 16 840 Inbar (1992) Shoval Israel 15 200 Inbar (1992) Gerar Israel 54 310 Inbar (1992) Jordan Israel 1,492 50 Inbar (1992) Meshushim Israel 160 20 Inbar (1992) Morocco 5,000 Heusch and Milles-Lacroix (1971) (b) Specific yields of the six highest yield northshore (European) and southshore (African) Mediterranean rivers from the database of Milliman and Syvitski (1992) from Woodward (1995) Northshore Mediterranean (Europe) Semani Albania 52. 4200 Shkumbini
















Milliman and Syvitski (1992) Milliman and Syvitski (1992) Milliman and Syvitski (1992) Milliman and Syvitski (1992) Milliman and Syvitski (1992)


Table 1 (continued) (b) Specific yields of the six highest yield northshore (European) and southshore (African) Mediterranean rivers from the database of Milliman and Syvitski (1992) from Woodward (1995) Northshore Mediterranean (Europe) Savio Italy 60. 1900 Southshore Mediterranean (Africa) Djer Algeria 3.9


El Harrach








Morocco 400.







Morocco 160.





Area 102 (km2)

Milliman and Syvitski (1992) Milliman and Syvitski (1992) Milliman and Syvitski (1992) Milliman and Syvitski (1992) Milliman and Syvitski (1992) Milliman and Syvitski (1992) Milliman and Syvitski (1992)

Reference Suspended sediment yield t km− 2 a− 1

(c) Mean annual suspended sediment yields for selected southshore river systems draining catchments in Mediterranean north-west Africa based on sources cited by Walling (1986) from Woodward (1995) Querrha Morocco 17.65 3590 Walling (1986) Aoudour Morocco 10.39 3850 Walling (1986) Sra Morocco 4.93 3500 Walling (1986) Allalah Algeria 2.95 4654 Walling (1986) Ebda Algeria 2.70 2493 Walling (1986) Leham Algeria 4.70 2028 Walling (1986) Agrioun Algeria 6.35 5300 Walling (1986) Fodda Algeria 7.67 4700 Walling (1986) Bou Algeria 5.75 270 Walling (1986) Namoussa Bou Algeria 11.65 88 Walling (1986) Hamdane Kebir Ouest Algeria 11.20 265 Walling (1986) Nebanna Tunisia 8.55 1330 Walling (1986) Kebir Tunisia 2.25 1313 Walling (1986) Kasseb Tunisia 1.01 5070 Walling (1986) Rhezala Tunisia 1.38 850 Walling (1986)

Laronne, 1995; Reid, 2002). Measurements of the dynamics of bedloads include those of Tacconi and Billi (1987) in Tuscany and the work of Sala and others in Catalonia (e.g. Sala, 1983; Batalla and Sala, 1995; Martin-Vide et al., 1999). Woodward (1995), in reviewing the patterns of erosion and suspended sediment yield for Mediterranean river systems, states that the region is naturally vulnerable to processes of erosion but it is the human contribution that is particularly significant. He calculates that around 75% of the sediment yield of Mediterranean headwater river basins may be attributed to human activity though


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Table 2 Rates of erosion measured on standard erosion plots throughout the Mediterranean Basin (from Wainwright and Thornes, 2004 and Kosmas et al., 1997) Cover



Mean annual soil loss t km− 2 a− 1


Agricultural plots Fallow/wheat rotation Wheat Barley – Wheat Durum Wheat Wheat Wheat Wheat Olives

Alentejo, Portugal Alentejo, Portugal El Ardal, Spain Potenza, Italy Tuscany, Italy Petralona, Greece Various Masquefa, Spain

1961–1991 1989–1990 1989–1992 1984–1987

65 plots 1983–1985

55–196 933–1015 33–99 133–200 260–1980 20–100 17.6 81–2400

Olives Vineyards

Various Duro, Portugal

3 plots 1979–1988

0.8 39


Duro, Portugal


Vineyards Vineyards Vineyards Vineyards Vineyards

1981 1981 1981

275–5385 210–1500 ≥7018 3255 32



Tropeano (1983)



Tropeano (1983)

Vineyards Vineyards Ploughed

Var, France Var, France Albugnano, Italy Mongardino, Italy Santa Victoria d’Alba, Italy Santa Victoria d’Alba, Italy Santa Victoria d’Alba, Italy Spata, Greece Various Aix-en-Provence, France

Roxo (1993) Roxo (1993) López-Bermúdez (1993) Postiglione et al. (1990) Chisci et al. (1981) Diamontopoulos (1993) Kosmas et al. (1997) Marqués (1991), Marqués and Roca (1987) Kosmas et al. (1997) Figueiredo and Ferreira (1993) Figueiredo and Ferreira (1993) Viguier (1993) Viguier (1993) Tropeano (1983) Tropeano (1983) Tropeano (1983)

1991–1992 9 plots 1965–1969

38–253 142.8 2220


Potenza, Italy



Kosmas (1993) Kosmas et al. (1997) Clauson and Vaudour (1971) Postiglione et al. (1990)

Alentejo, Portugal



Roxo (1993)

Alentejo, Portugal



Roxo (1993)

El Ardal, Spain



López-Bermúdez (1993)

Tabernas, Spain Aisa, Central Pyrenees, Spain Aisa, Central Pyrenees, Spain Aisa, Central Pyrenees, Spain Aisa, Central Pyrenees, Spain Aisa, Central Pyrenees, Spain Spata, Greece

1991–1992 1990–1991

16–40 51

Puigdefábregas (1993) Garcia-Ruiz et al. (1995)



Garcia-Ruiz et al. (1995)



Garcia-Ruiz et al. (1995)



Garcia-Ruiz et al. (1995)



Garcia-Ruiz et al. (1995)



Kosmas (1993)

Nahal Yael, Israel Alentejo, Portugal Alentejo, Portugal El Ardal, Spain Tabernas, Spain

1970–1971 1988–1991 1989–1990 1989–1992 1991–1992

3–18 81 217 32 6

Yair and Klein (1973) Roxo (1993) Roxo (1993) López-Bermúdez (1993) Puigdefábregas (1993)

Vineyards Vineyards

Abandoned and ‘natural’ plots Abandoned agricultural land/natural vegetation Abandoned agricultural land/natural vegetation Fallow land with rock fragments Abandoned agricultural land Abandoned, dense shrubs Abandoned-grass and open shrub canopy Abandoned — low cover Abandoned, stone pavement Abandoned, bare Olive grove with extensive grass cover Debris slope Cistus Cistus Natural matorral Natural vegetation, retama

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Table 2 (continued) Cover



Mean annual soil loss t km− 2 a− 1


Abandoned and ‘natural’ plots Natural vegetation, anthyllis Natural vegetation, stipa Matorral Matorral Matorral Matorral

Tabernas, Spain Tabernas, Spain Sierra Valencia, Spain Murcia, Spain Murcia, Spain Aix-en-Provence, France

1991–1992 1991–1992 1988–1989 1985–1992 1985–1992 1965–1969

22 34 9–21 39 0.6 0.5

Natural vegetation Shrubland Burnt matorral Maquis Beech woodland Evergreen oak forest Eucalyptus plantation Eucalyptus Bare Bare Bare

Santa Lucia, Italy Various Santa Lucia, Italy Petralona, Greece Santa Fe, NE Spain La Castanya, NE Spain Santa Lucia, Italy Various Sierra Valencia, Spain Tuscany, Italy Basilicata, Italy

1992–1993 95 plots 1992–1993

34 6.7 31 25–218 152 203 66 23.8 5–16 1347 Up to 28 000

Puigdefábregas (1993) Puigdefábregas (1993) Rubio et al. (1990) Albaladejo et al. (1991) Albaladejo et al. (1991) Clauson and Vaudour (1971) Aru and Baroccu, 1993 Kosmas et al. (1997) Aru and Baroccu, 1993 Diamontopoulos (1993) Sala and Calvo (1990) Sala and Calvo (1990) Aru and Baroccu, (1993) Kosmas et al. (1997) Rubio et al. (1990) Pannicucci (1972) Rendell (1982)

1982–1985 1982–1985 1992–1993 12 plots 1988–1989

Source: After Wainwright and Thornes (2004) and Poesen and Hooke (1997).

Bintliff (2002) disputes that interpretation. Woodward produced graphs of intensity of erosion for each of the Mediterranean countries and figures of sediment yields for major rivers (Table 1). Likewise, Wainwright and Thornes (2004) produced figures of rates of erosion based on rates of sediment transport measured at the catchment scale (Table 1). Many authors produced data on reservoir sedimentation, showing rapid rates of filling. Conacher and Sala (1998) consider that sediment supply to reservoirs in parts of Spain is amongst the highest in the world. The influence of vegetation and particularly of agriculture is shown in the various compilations of erosion rates, mostly drawn from plot studies (e.g. Poesen and Hooke, 1997; Wainwright and Thornes, 2004) in which rates on agricultural land are mostly higher than on abandoned and more vegetated or semi-natural land (Table 2). 3. Impacts of human activities 3.1. Historical human impacts and fluvial chronology The extent of woodland, the phases and effects of clearance, the causes of soil erosion and relationship to civilisations and to climate in the past have all been much debated. Darby (1956) considered that much Mediterranean woodland may have been quite open in the driest parts of the region and provided evidence of dense woodland in ancient Greek times (e.g. Homer in 9th century BC describing densely wooded areas) but

also of the considerable activity of woodcutters, particularly for shipbuilding. It was considered that forests could not regenerate because of goats. The prehistoric and historical context of ecological changes of the European Mediterranean were analysed by Butzer (1972) and were reviewed more recently by Grove (1996). Roberts (1989), in a synthesis of Holocene changes, shows that vegetation disturbance was clearly distinguishable in pollen after 2000 BC. From the Bronze Age in Greece onward, the Mediterranean triad of wheat, olive and vine are present. The impact of clearance and agriculture varied, however, in timing; for example, cedar forests persisted in Morocco until recently. Wainwright and Thornes (2004) also debate the cultural influence on vegetation. Their graph of probability of anthropogenic disturbance based on pollen and 14C dating shows large peaks around 700 AD (Fig. 2). They conclude (p. 266) that ‘The slow but sure transition to agriculture… over a period of about six thousand years was one that produced the most significant single impact on the landscape. It led to major changes in the hydrological cycle and caused rates of erosion increase by orders of magnitude. … Initial impacts were localised and minor’ but changes in the Bronze Age became more widespread. The development and expansion of agriculture was by no means continuous and periods of decline, for climatic and socio-economic reasons, are identifiable. For example, Atherden and Hall (1999) correlate vegetation changes in mountains of Crete since 500 AD with demographic,


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Fig. 2. Cumulative probabilities of calibrated radiocarbon dates for recorded events of first occurrences of various forms of anthropogenic modification of vegetation assemblages within the Mediterranean as a whole (after Wainwright and Thornes, 2004).

political and cultural affairs of the island. In summary, major deforestation and clearance of land for cultivation of primarily wheat, olive and vines and for pastoralism of goats and sheep took place from early times in parts of Mediterranean Europe, and at later times elsewhere. Widespread forest clearance was attributed by many as the cause of major soil erosion and land degradation and the subsequent aggradation of river valleys. Alluvial stratigraphy provides evidence for very significant changes in river regime and channel morphology in parts of the Mediterranean. The early classic work on this was by Vita-Finzi (1969) who recognised two major phases of aggradation. The earlier one is Pleistocene but the Younger Fill is late Roman — early medieval (250–1450 AD), formed from eroded topsoil. Extra sediment reached the sea, choking some harbours, e.g. at Ephesus. Much more recent work has been done and the original Vita-Finzi simple and generalised chronology for the whole Mediterranean has now been shown to be much more complex (Macklin et al., 1995). Research by Van Andel et al. (1990) appeared to support effects of human activities and later research showed human impacts from early Neolithic (6500 BC) onwards. On the other hand, Grove (2001) argues that the fluvial terraces in Mediterranean Europe known as Younger Fill could be better explained by increased frequency of deluges, correlated with glacier advances so they are climatic, rather than soil erosion resulting from deforestation. A key debate has emerged on causation between proponents of Vita-Finzi and van Andel, which is discussed by Bintliff (2002). He investigates the natural and human interaction and the possibility of multiple causes, including the role of natural floods in moving the products of soil erosion from human impacts. Vita-Finzi's identification of restricted episodes of erosion and alluviation remains an important idea. Ballais (1995) illustrates the complex interaction of climatic and human influence from

alluvial sequences in the Maghreb. Similar kinds of sequences are detectable in alluvial fans, with major phases of alluviation but much dissection during the Holocene, attributed dominantly to climatic influence but with some human impact (e.g. Calmel-Avila, 2002; Harvey, 2002b). Much debate also exists in the literature about the origin and rates of erosion of badlands (Torri and Bryan, 1997). Some evidence indicates that some badlands are of very recent development and of human origin, (e.g. Calzolari et al., 1997; Torri et al., 2000), others natural, and that some may be very ancient (Wise et al., 1982; Woodward, 1995). Bintliff (2002) similarly challenges previous anthropogenic interpretations of causes of deltaic sedimentation in the Mediterranean. The complexity of possible interrelations now revealed by combinations of geomorphological and archaeological study have important implications for our interpretation of more recent impacts and alluvial changes, though it is difficult to make comparisons between current and past episodes because of changed conditions. Bintliff (2002; p. 431) concludes that ‘Current indications suggest that a more interactive human ecological model involving the convergence of semi-autonomous anthropogenic and natural processes is the approach most in agreement with the current state of knowledge concerning Mediterranean alluvial phenomena.’ 3.2. Water and river management The water deficit in summer has meant that a long history of water management occurs in much of the region. Debate on when irrigation was first introduced to the European Mediterranean region suggests that it may possibly have been as early as Bronze Age times (c. 5500 BC); for example, Gilman and Thornes (1985) cite evidence of irrigation at Antas in SE Spain at 2000 years BC. Early hydraulic civilisations existed in

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the region and large-scale schemes for water transfer and irrigation were developed in Roman times, as witness many surviving Roman aqueducts. The Moors also had sophisticated hydraulic systems that they introduced to southern Spain, including the system of qanats (Butzer et al., 1985; Lightfoot, 1996). Irrigation and control of water supply was also a major factor in the development of some early cities (Wainwright and Thornes, 2004). A large variety of ancient and traditional water harvesting systems have been used in the Mediterranean region, some involving direction of runoff towards crops, others using various kinds of storage. Roman methods of dry farming in Tunisia, for example, enabled it to become the ‘bread basket’ of the Empire. Gilbertson et al. (1994) describe ‘the profound environmental changes on wadi floors and closed basins’ brought about by floodwater farming in Tripolitania (Libya) in about the 1st century AD, in which a highly intricate system of walls and structures controlled erosion and sedimentation and was able to sustain mixed farming. Some of these structures still have an influence on processes. In southern Spain various systems of water supply to fields are used in traditional dryland farming systems on valley floors. Hooke and Mant (2002a) have analysed the hydraulics of some of these surviving systems and show that they can take off a significant proportion of small to medium flows in storms. More elaborate canal systems also exist which must have entailed considerable investment, and imply frequent enough channel flow to justify construction, though most of these are now falling into disrepair or are no longer functional because of groundwater pumping in the area. Large-scale transfers of water have been developed in modern times. In southern Spain the construction of a transfer canal and pumping system from the River Tagus in central Spain to Murcia province in the southeast allowed enormous expansion of irrigation and intensive agriculture, as intended. It has also allowed the expansion of tourist resorts and residential areas, with the present phenomenon of a large increase in the number of golf courses that is creating further demands on water. This has led to search for new sources, because aquifers are also much depleted. The formation of the Spanish Water Plan., proposing the transfer of water from the Ebro in NE Spain, was approved by the Spanish Parliament in 2004. However, this plan was immediately rescinded by an incoming Spanish Government and proposals of how to deal with the water crisis in SE Spain were again under discussion at the time of writing. Impoundments date from ancient times in the Mediterranean region but the number and size of dams has increased almost exponentially over the last


100 years (Woodward, 1995). Poulos and Collins (2002) estimate that over 3500 dams, both small (>= 30 m) and high (>= 60 m), exist in Mediterranean countries, with over 98% of these constructed since 1800 and 84% within the last 50 years. Using data for the 169 largest rivers in the region, they calculate that these dams have reduced the sediment yield into the Mediterranean Sea to 35% of that before dam construction (Table 3). The high sediment yields and high retention rates have produced major problems of rapid infilling of reservoirs. Construction of check dams, mainly on small tributary channels, has also had a major impact on the fluvial systems in the region. These check dams vary in size and construction materials. They cause sedimentation and increased vegetation upstream, and produce scour downstream. However, they are prone to collapse, even in moderate floods (Hooke and Mant, 2002a), and collapse or filling reduces the effect on the sediment dynamics in the longer-term. It is often difficult to get details of this scale of management but, for example, Liebault and Piegay (2002) have documented historical works in pre-Alp catchments and attribute major incision, particularly in the 20th century, to such works. The mining of gravel from channels is now outlawed in many countries or areas but some still continues, legally or illegally. Gravel extraction has had a profound effect on rivers, particularly in Italy (Surian and Rinaldi, 2003, 2004) and France (Liebault and Piegay, 2002) in recent decades and is also a major contributor to river incision, especially on streams where sediment supply has now been reduced by dams (see Section 4). Massive direct transformation of many channels, mainly by straightening and embankment, has been undertaken for flood control and for increased use of floodplains, and, on the bigger rivers, for navigation. In many cases, formerly wide, braided and unstable river courses have been constrained into single, narrow deep channels. For example, on the Po River in Italy embankment began in the 12th century and the river was completely embanked by the 19th century (Marchetti, 2002). Braga and Gervasoni (1989) provide detailed historical evidence of canals and river modification dating from medieval times. The dates of channelisation vary between rivers but, as well as the direct modification, the alterations prevent the river from responding in the same way to other land use and catchment changes. The effects depend on river regime and sediment supply but, generally, the lack of reworking of floodplain alluvium has meant lack of sediment downstream. On the other hand, the narrowing and straightening of the river removes locations for channel storage on rivers with high amounts of sediment so aggradation within


J.M. Hooke / Geomorphology 79 (2006) 311–335

Table 3 (a) Numbers of dams in the countries around the Mediterranean Sea (Abstracted from ICOLD (1998), Poulos and Collins (2002)) Prior 1900


Construction Periods 1950–1999 1950–1959

Albania Algeria Bosnia Croatia Cyprus Egypt France Greece FYROM⁎ Italy Lebanon Libya Morocco Spain Syria Tunisia Turkey Yugoslavia Total


1 3 3 5 5

73 9 9 4 16

3 1 0 0 0 35 0 0 6 0

15 0 0 0 4 125 1 1 190 0

75 3 2 116 2

90 4 8 77 3

0 52

7 215

6 159

5 211 13

6 10 396

29 10 561


3 5 Subtotal 566

1970–1979 125 6 4 4 4 1 93 3 3 43 0 8 8 195 6 72 11 586 2991



98 40 7 14 21 1 96 7 2 33 0 4 37 186 7 10 151 27 720

9 31 1 2 6 55 26 31 0 29 139 6 61 339 2 728

(b) Dammed areas and sediment fluxes, before and after damming Poulos and Collins (2002) Physiographical region

Region 1 Region 2 Region 3 Region 4 Region 5 (excl Nile) Region 5 (incl Nile) Total

Dammed area (103 km2) (Approx)

Percentage of dammed to overall catchment area (%)

Before damming

After damming

SS flux (106t)

Total sediment flux (106t)

SS flux (106t)

Total sediment flux (106t)

396 129–165 136 187

138.9 41.2 67.5 26.5

174 51 84 33

c.225 c.31 c.125 c.145

46 51 37 70

257.2 84.0 107.1 88.2






9.7 1012

283.8 (43%)

12 355 (35%)

⁎FYROM: Former Yugoslavia Region of Macedonia.

the channel can occur, as in the case of the Po. The presence of structures for channelisation can also influence the impacts of major floods, as shown in the case of a catastrophic flood in the Aude valley in 1999 (Arnaud-Fassetta et al., 2002) in which current land use and valley management is alleged to have intensified the flood. 3.3. Indirect impacts — land use change As briefly reviewed, major deforestation and clearance of land for cultivation of primarily wheat, olive and vines and for pastoralism of goats and sheep took place from early times in parts of Mediterranean Europe, and at later times elsewhere in the region. Widespread forest

clearance was attributed by many as the cause of major soil erosion and land degradation and the subsequent aggradation of river valleys. Conacher and Sala (1998) review the effects of clearance and agricultural practices and provide evidence of present soil erosion and aggradation of river channels and Kosmas et al. (2002) and others provide data on rates of erosion under different crops (Table 2). A major characteristic of much of the European Mediterranean and part of the traditional agriculture and cultural landscape is the presence of agricultural terraces. Some of these are very narrow (< 3 m wide) and built on very steep slopes They were constructed at various times, including in Roman times and the Moorish period and with major phases in the 18th and

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19th centuries in parts of the region, to provide areas of flatter land on slopes and to retain soil and moisture, Some have stone retaining walls and others have earth embankments. Many of them are no longer being maintained and so are major hotspots of erosion where walls or banks are breached by overflow. They are also commonly locations of large-scale piping because of ponding of water and physical and chemical processes in vulnerable materials. Data on variation in erosion rates on hillslopes in relation to crop type and method of farming indicate that rates of erosion are highest in vineyards and on ploughed land but are also high under modern methods of cultivation of tree crops (Poesen and Hooke, 1997; Kosmas et al., 1997, 2002; Loughran et al., 2000). Serrat and Ludwig (2004) compare two catchments in the Agly basin, France, one under viticulture and garrigue, the other forest. Much higher sediment loads, both of suspended and bed load, are produced from the former, and they conclude that anthropogenic factors must be dominating because the second catchment has steeper slopes. Kosmas et al. (1997) explained low rates under olives as usually being on old terraces and often not ploughed. Changes in land use practices, such as methods of ploughing, can also make a difference, e.g. Bazzoffi et al. (1986) quantified erosion rates from two contrasting agricultural areas in Tuscany using plot measurements and 137Cs in reservoirs. Soil losses were much higher from plots with surfaces smoothed by machines than unsmoothed plots under various crops. In the early 20th century many parts of the Mediterranean still had a traditional agricultural and rural way of life but then a phase of land abandonment occurred for various socio-economic reasons, particu-


larly in Spain and Portugal, mainly in the period 1950– 1980. Much research is ongoing into the effects of abandonment and how vegetation recolonises (e.g. Ruecker et al., 1998; Obando, 2002). Land abandonment can have positive or negative effects on land degradation depending on the ability of land to recover (Conacher and Sala, 1998). Much depends on the extent of regrowth, controlled by rainfall, whether grazed, and the incidence of fire, which itself is related to biomass. An initial phase of greater erosion may result from the degraded soil condition and lack of organic matter after cultivation. Gallart et al. (1994) examined the hydrological functioning of terraces and how areas of saturation develop but that changes since abandonment of terraces can lead to reorganisation of the network. Similarly, Llorens et al. (1997) and Lasanta et al. (2001) examined changes and found that sediment transfer could increase after land abandonment. However, the general assumption is that vegetation cover tends to increase and erosion rates decrease some years after abandonment. Mazzoleni et al. (2004a) document case studies of changes in vegetation cover in the Mediterranean, mostly derived from remote sensing, many of which demonstrate a large overall increase in cover, much of it attributed to land abandonment. Osborne and Woodward (2001), however, consider that increased vegetation cover documented from remote sensing may be due to the effects of global warming. EU policies and subsidies are now leading to cultivation of particular crops in Europe, as exemplified by recent trends in SE Spain (Fig. 3). This is the cause of the large expansion of almonds, grown on steep slopes with ploughing, often up and down slope, which has resulted in increased rilling and gullying (e.g. Faulkner,

Fig. 3. Changes in land use in Salada catchment, Murcia, Spain, 1956–1997 (after Lopez-Bermudez, 1999; Hooke and Mant, 2002a).


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1995; Oostwoud Wijdenes et al., 2000; Poesen et al., 2002, 2003). Tillage erosion is also calculated to have increased in recent years because of changes in mechanisation and ability to deep plough and because of the expansion of almond groves on steep slopes (Poesen and Hooke, 1997). As well as a change in crop type within the European Mediterranean zone major changes in methods of agriculture have taken place in recent decades. Some

of this has involved complete remodelling (land levelling) of the landscape, removing badlands, alluvial fans and steep slopes. It is arguable that this topographic change is on an unprecedented scale (but the extent and degree of agricultural terracing in former times was also very large). What is happening now, especially in SE Spain, which has become the provider of horticultural products to the supermarkets of Britain and Germany, is an industrialisation of agriculture on a massive scale.

Fig. 4. A. Map of part of the Orcia catchment, Tuscany, Italy, showing extent of remodelled land (derived from Busoni et al., 1995). B. Photograph of biancane (badland) and remodelled pasture land, Orcia valley, Tuscany. C. Photograph of calanchi (gullies) and remodelled slopes, Orcia valley, Tuscany.

J.M. Hooke / Geomorphology 79 (2006) 311–335


Fig. 4 (continued).

Over large parts of the coastal margins of Murcia and Almeria provinces the former semi-natural areas of the 1960s and 1970s have been bulldozed and flattened to produce very large areas on which plastic greenhouses are constructed for growing tomatoes. In other areas flat fields have been created in which crops of lettuces, broccoli, melons and such like are produced by drip irrigation. Many of the areas used for the greenhouses are alluvial fans. Calcrete is cleared from the surface to expose underlying weathered material. The crops are then grown using massive inputs of organic matter and nutrients. Many regard this type of agriculture as completely unsustainable and water is already running out. The drainage lines and topography are completely disrupted. In many cases the soil and rock material is

completely reconstituted and all natural profiles lost (Borselli et al., 2006). Varela et al. (2001) studied soil properties after deforestation and after land levelling in temperate-humid NW Spain and found that the latter had greater effect and that soil mechanical strength did not show recovery. Documentation of the effects of this industrialised agriculture, however, is still limited. The extent of soil erosion, prior to this latest phase of alteration, was such that many areas in southern Europe had become deeply gullied and a landscape of localised badlands had developed. For example, in Tuscany, in the area south of Siena, the occurrence of the rounded forms of badland called ‘biancane’ (Fig. 4B), as well as gullies (‘calanchi’) (Fig. 4C), had become typical. Now the land has been transformed by landscaping and


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bulldozing. The large scale of this is shown by a map from an area of the Orcia Valley in Tuscany, (Fig. 4) and also by examples in southern Italy (Clarke and Rendell, 2000). Formerly deeply gullied and badland areas have been transformed into large smoothed fields which are used for grazing sheep or growing wheat (Fig. 4B and C). The transformation is such that some areas of biancane are now being conserved as an endangered landscape feature. This is ironic in some ways because at least some of the biancane have resulted from agricultural practices within recent centuries (Calzolari et al., 1997). This raises questions of the desirable landscape and the timescale perspective for management in such a culturally modified region. The problems of soil erosion meant that a major phase of afforestation took place in Mediterranean uplands in many countries during the 20th century. Most forestry systems used terracing and homogeneous stands of Pinus species but the efficacy of those policies and methods is now beginning to be questioned. For example, in southeastern Spain questions arose in part because it is difficult to grow trees there and trees are not regenerating in the ways expected. Moreover, the trees may not prevent erosion because of insufficient ground cover, and the existence of terraces produces concentrated erosion. Navarro et al. (2005) show that the landscape setting and overall topography cannot be negated by the presence of the terraces. The trees were also grown to prevent flash floods in catchments but Lopez-Bermudez et al. (2002) briefly review legislation in Spain and comment that the afforestation policies still failed to prevent major floods. Sorriso-Valvo et al. (1995), in a case study of afforestation in Calabria, show that the effects on hydrology and erosion may be varied and complex. Forest policy, however, may also be tied up with occurrence and effects of fire. Fire can be of natural or human origins. Stewart (1956) describes the lack of wood for the Romans by the fifth century and that fire contributed to the destruction of forest. In the modern era, Conacher and Sala (1998) provide evidence that fires are increasing in area affected, trebling in 30 years from 1960–1990, and becoming more frequent. Wainwright and Thornes (2004) outline the effects of fires and discuss the rethinking of attitudes in relation to vegetation. Forest cover increases fire hazard and it was formerly thought that fires should be avoided. It is now thought that fires can add to genetic diversity and stability and groupings of species. Wainwright and Thornes (2004) tabulate modified rates of erosion and show that the benefits are mostly from low intensity fires and that high intensity fires can be destructive.

The timing of major storms in relation to fire influences the impact on fluvial systems. Another major human impact in Mediterranean regions, as elsewhere, is that of urbanisation. Wainwright and Thornes (2004) review the historical development of urban areas in the Mediterranean region and the interaction with rural land use. Major towns have been built on rivers, often with associated schemes for flood prevention, but waves of town expansion are apparent at different times. Land abandonment in the1950s meant large movements of population to cities and now a new phase of urbanisation is occurring in southern Spain, much of it in formerly rural areas. Amongst the few specific studies of fluvial impacts of urbanisation in the Mediterranean region is that of Sala and Inbar (1992), in Catalonia. Urban area increased by 3–35% in different catchments and is thought to be the cause of the 50% increase in runoff ratio recorded. Lag times decreased and the increase in peak discharge was three times larger in urbanised than non-urbanised areas. An increasing frequency of devastating floods is also documented. The causes of catastrophic flooding are attributed elsewhere to increased runoff from increased impervious areas in expanding settlements, for example, in the Aude catchment in France (Arnaud-Fassetta et al., 2002). 4. Recent fluvial changes and impacts Having examined the impacts and trends in individual human activities, the overall impacts and changes in fluvial systems can be assessed. The impacts of human activities are often difficult to disentangle from those of climate fluctuations and debate continues on the relative importance of each. As described, major phases of deforestation and excess sediment loads took place early in Europe but Conacher and Sala (1998) describe current conditions of sedimentation in many small catchments because of excessive supply and that increased flood hazard is occurring because of aggradation. They present evidence that phases of deforestation and increases in land degradation are still prevalent in North Africa. A substantial body of literature now documents a recent phase of narrowing and incision of rivers, in many parts of the Mediterranean but especially in France and Italy. Liebault and Piegay (2002) discuss the narrowing of channels in SE France since 1850 and produce a conceptual model of factors influencing channel narrowing. They discuss the problems of establishing causation but provide details of torrent control works including reforestation, check dams, fascines and brush gully checks. They provide evidence of a general trend of narrowing from 1850 but accelerated, strong narrowing

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in the period 1950–1970; narrowing averaged 55% in the period 1948 to c.1991 on various tributaries. Channels have changed from braided to wandering or meandering and width changes are similar on the large downstream rivers and the mountain streams. The longterm trend since 1850 could be due to climatic changes from the end of the Little Ice Age and a recovery process from destabilisation by the floods in basins which were highly responsive after human disturbance. The recovery, however, is likely to have been accelerated by the torrent control works which reduced sediment supply to the channel and allowed vegetation establishment along channels. The recent phase is human-induced because of floodplain afforestation and abandonment of land in alluvial sections, further reducing sediment load. Surian and Rinaldi (2003) provide an overview of channel adjustments of Italian rivers over the past 100 years. Two main types of change are apparent,


incision of 3–4 m, and narrowing of channels by 50% or more. These are attributed to sediment extraction, dams and channelisation causing decreases in sediment supply. Adjustment was rapid at first then asymptotic. Surian and Rinaldi produced a model of changes, showing that formerly braided channels tend to narrow, and single channels become incised. Surian and Rinaldi (2004) provided detail on the impacts of reforestation, torrent control works, levee construction, dams, and sediment mining on five rivers of northern Italy (Table 4). Width reduction over the past two centuries has been 58–85%. However, channel widening, of the order of metres per year, began again in the 1990s, which they attribute to exhaustion from the previous adjustment. Marchetti (2002) has documented similar effects on the Po River, as well as reviewing the longer Holocene chronology. The mining of gravel from the channels was particularly severe in the period 1950–1970. Impacts of

Table 4 (a) Human interventions and adjustments in selected rivers in Italy (after Surian and Rinaldi (2004)) River

Drainage area Upstream from dams (%)

Time of dam closure

Time of intense sediment mining

Construction of levees and other protection structures

Reforestation and torrent control works in the drainage basin

Tagliamento Piave Brenta Trebbia Vara

3 54 40 25 36

1950s 1930s–1950s 1954 1920s–1950s 1930s

1970s–1980s 1960s–1980s 1950s–1980s 1950s–1980s 1960s–1970s

19th–20th century 14th–20th century 19th–20th century not considerable 20th century

20th century (?) since the 1920s since the 1920s not considerable 20th century

(b) Channel narrowing in the selected rivers (after Surian and Rinaldi (2004)) (Narrowing = Ng) River

Total Ng (m)

Total Ng (%)

Ng in 1st phase (% of tot.)

Ng in 2nd phase (% of tot.)

Rate of total Ng (m y− 1)

Rate of Ng in 1st (m y− 1)

Rate of Ng in 2nd (m y− 1)

Tagliamento Piave Brenta Trebbia Vara

942⁎ 580⁎⁎ 276⁎ 403⁎ 569⁎

58 69 58 62 85

46 25 33 56 58

54 75 67 44 42

5.0 6.5 1.4 2.2 3.3

2.8 2.4 0.6 1.6 2.5

13.2 14.6 4.2 4.2 5.7

(c) Channel widening in the selected river (after Surian and Rinaldi (2004)) (Widening = Wg) River

Wg (m)

Period of documented channel wg

Wg compared to original channel width (%)

Wg compared to previous channel width (%)

Rate of g (m y− 1)

Tagliamento Piave Brenta Trebbia Vara

72 49 21 4 13

1933–2001 1991–1997 1999–2002 1996–2000 1996–2000

4 6 4 0.6 2

11 19 10 2 13

9.0 8.2 7.0 1.0 3.3

Total narrowing: refers to the period from early 19th (⁎) or 20th (⁎⁎) century to the 1990s. Narrowing in the 1st phase: refers to the period from early 19th or 20th century to the 1950s. Narrowing in the 2nd phase: refers to the period from the 1950s to the 1990s. Rate of narrowing: average rate of narrowing estimated over the different time periods considering initial and final channel widths. Widening compared to original channel width: represents the ratio between the amount of widening and channel width in the early 19th or 20th century. Widening compared to previous channel width: represents the ratio between the amount of widening and channel width at the beginning of the widening process. Rate of widening: average rate of widening estimated over the different time periods considering initial and final channel widths.


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human activities can be further complicated, as Uribelarrea et al. (2003) have shown in a study of the Jarama and Tagus Rivers, Spain, because certain kinds of human interaction can alter the responsiveness of the channel to other changes. Formerly, a close relation existed between river response and climate, and between flooding and channel changes in historical times on these rivers., After 1950, when dams and gravel mining took place, natural development and adjustment of the river channels was impeded. 5. Spatial patterns and connectivity Overall, the historical and recent patterns indicate that human activities have had a profound effect on the fluvial systems, from hillslope runoff and erosion through to delivery of sediment to the coast. Positive feedback effects operate once land is cleared and land degradation begins, with rapid acceleration of decline in infiltration rates, thinning of soil, and increase in erosion. Likewise, once incision, riparian revegetation and detachment from floodplains are initiated then this can accelerate by positive feedback because of lack of sediment supply. The actual impacts are spatially and temporally varied and it is suggested that the connectivity within different parts of the system plays a major control on the extent to which changes are propagated and transmitted down through the system. Cammeraat (2002, 2004) has examined this concept at a hierarchy of scales and has demonstrated from field examples in SE Spain the influence of connectivity patterns on water and sediment delivery on hillslopes. In channel systems, the importance of connectivity has been discussed by Hooke (2003) and is exemplified for a channel in SE Spain. Brierley and associates (Brierley and Murn, 1997; Fryirs and Brierley, 1999, 2001; Brierley and Stankoviansky, 2002; Brooks and Brierley, 2002; Brierley et al., 2005) have contributed significantly to the concepts and have demonstrated how the fate of sediments eroded after European colonisation and disturbance in eastern Australia and location of storage are influenced by connectivity. These examples illustrate the importance of understanding the functioning of different channel reaches and the ability to transport sediment and propagate changes downstream, a fundamental point also made by Harvey (2002a). This has been a missing element in some statistical analyses of sediment yield relations in catchments. The connectivity depends on the extent of discontinuities and buffers in the system, which may be natural, or anthropogenic, and the nature of linkages and coupling. A key to understanding is to examine spatial locations

and patterns of interference and land cover and to relate this to connectivity of runoff and sediment within the system at scales ranging from patch to catchment. The effects will be exemplified by looking at various parts of the fluvial system, working progressively from hillslopes and source areas, to hillslope–channel coupling, to channel transmission, and finally to impacts on coastal delivery of sediments. An example of alterations on hillslopes, cited earlier, is that of the removal of badlands and gullying (biancane and calanchi) in Italy by bulldozing to create smooth slopes, completely relandscaping the hillsides (Fig. 4). Few studies of the impacts of this have yet taken place but Clarke and Rendell (2000) show that remodelling badlands creates longer slopes at lower angles. It has altered spatial coupling of soil erosion processes, which were formerly localised in badlands, by connecting the slopes. Although apparently decreasing local rates of slope erosion in the short-term, it means eroded material is no longer deposited at the base of the badlands slopes but continues downstream. Many expect erosion to increase again in the longer-term, though much of the topography is likely to be maintained by simple rebulldozing. A major impact in much of the European Mediterranean area is the abandonment of land and the neglect of old agricultural terraces or the direct removal of terraces to allow much more highly mechanised and intensive crop cultivation, particularly almonds, on steep slopes. This also involves direct up and down slope ploughing. Faulkner (1995) has studied the effects of this in almond groves in southern Spain. She distinguished two types of gullies which tend to develop, one under Hortonian overland flow conditions on schist on the upper slope and the second developing on mid-slope as large hammer-head gullies influenced by the presence of colluvium and the morphology of the slope. Incidence of gullies was much higher under the almonds than on the old maintained Moorish terraces. These gullies are connecting down slope and delivering sediment to the base of the slope and into the channels, so that most of the current sediment yield is coming from these highly localized sites. Likewise, Gallart et al. (1994) suggested that decline of the terraces gives rise to reorganisation of the network, i.e. an alteration of the pattern and degree of connectivity. The influence of terrace structures and the nature of contributory areas on the pathways of flow on the development of gullies are illustrated from an area of steep slopes and terraced fields near Murcia, Spain. The gully in Fig. 5a was created in a storm of September 1997 (Hooke and Mant, 2000) with the major pathways coming from breached terraces upslope.

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Fig. 5. a. Photograph of a gully developed during September 1997 storm in Salada catchment, Murcia, Spain. b. Location of gully and pathways of runoff at Salada gully site; dashed line = 1997 pathway, dotted line = 2004 pathway.

This gully was bulldozed and remade by the farmer shortly afterwards. In 2004 the gully partially reformed, however, this time with more runoff coming from the bare slope on the right (Fig. 5b). Similarly, a mapping of flow and sediment pathways and of erosional and depositional areas following a small storm in a subcatchment of Carcavo, northern Murcia province, Spain, shows the localised influences of runoff-producing areas, structures altering flow paths, and the absence of vegetation on erodibility (Fig. 6). Although in both of these cases the terraces themselves are flat, high hydraulic gradients are present overall because of deeply incised river channels below the slopes. The importance of the specific lines of connectivity as represented by gullies in influencing the supply of

sediment down slope and to channels is emphasised by Poesen et al. (2002, 2003). They consider that gullies are the major sources of sediment production in Mediterranean areas and make a much higher contribution than rill or sheet erosion. Poesen et al. (1996) calculated that ephemeral gullying produced 80–83% of the sediment from cultivated lands in southeastern Portugal and rangelands of southeastern Spain. Verstraeten et al. (2003) studied sediment yields into reservoirs in Spain and found that yields did not correlate well with catchment area or with a combination of climatic, topographic and land use factors. Results were improved by the addition of a gully factorial scoring and by consideration of the area immediately around the reservoir.


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Fig. 6. Piping and incipient gullies developing in almond groves, Carcavo catchment, Murcia, Spain. Lines indicate pathways of flow and connectivity from terraces.

Within small headwater and tributary channels a major and deliberate effect on the connectivity is the construction of check dams or torrent control structures of various kinds. This practice is very widespread throughout the Mediterranean region and is a major plank of catchment management policy in many countries. The scale of the impact is illustrated from the Carcavo channel in northern Murcia province, Spain. The long profile of the channel has been surveyed using accurate differential GPS (Fig. 7) and demon-

strates the high number of check dams present and the amount of storage on this steep stream, amounting to about 38% of this length of channel being affected by storage zones. Most of these structures are very recent yet most are already full of sediment. Several have already collapsed and this is common, so the effects vary with timescale. In the short-term they drastically reduce the sediment load and supply downstream, but within decades they tend to fill up and then provide a focus of increased energy at the ‘waterfall’ created. Collapse of

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Fig. 7. Long profile of Carcavo channel, Murcia, Spain, showing number and spacing of check dams and length of sedimentation zones.

the structures can exacerbate the catastrophic effects of a flood, as seen in the Biescas disaster in the Pyrenees (Garcia-Ruiz et al., 1998). The resulting flood wave tends to sweep out all sediment for a distance downstream, eventually increasing the delivery to lower parts of the system. Within channels, structures can have a significant localised effect, for example on location of scouring. This was illustrated in the effects of a moderate flood in the Torrealvilla catchment in SE Spain in September 1997 (Hooke and Mant, 2000). Mapping and resurvey of channel reaches showed that the highest amounts of scour were just downstream of country roads, where sediment transport was impeded. In large floods, such structures would be washed out. The localised scour locations can, however, be the initiation of knickpoints that are transmitted upstream and can lead to wider scale degradation. Warner (2006) has also demonstrated the influence of anthropogenic structures such as embankments in creating discontinuities within the channel and floodplain of the Durance River. These structures influence constraint of floodwater, the hydraulics of flow and the location of deposition. The effects of the sediment supply and erosion of hillslopes and channel margins on the delivery of sediment to the coast are illustrated by the variations in some of the major deltas of the Mediterranean area. Various phases of development have occurred in the past but a major phase of erosion is now occurring because of decreased sediment loads to the coast. Poulos and Collins (2002), in a study of 169 of the major rivers draining to the Mediterranean Sea, calculate that sediment supply has been decreased to 35% of formerly,

mainly due to the construction of dams. The idea of high sediment delivery, however, is a matter of timescale and period of study. Vita-Finzi (1975) considered that large parts of the Po delta had accumulated since the late 18th century and that many deltas of the southern Spanish coast did not form until the 15–16th centuries. Poulos and Collins (2002) indicate historical rates of progradation of metres per year for Mediterranean deltas and current erosion of tens of meters per year on several deltas (excluding the Nile which is even higher). This again raises questions on what should be regarded as ‘stable’, ‘normal’ or acceptable in the long-term. These major human barriers, dams, have reduced the channel connectivity in recent decades. 6. Discussion and implications Analysis of the human influences in the Mediterranean region shows some major trends, with early deforestation and clearance leading to massive increases in soil erosion and supply of sediment to rivers and the coast. More recent phases of damming and control of rivers, some afforestation and reforestation or revegetation have lead to decline in sediment loads, incision of rivers and depletion of supply to coasts. These effects were, however, probably mediated by climatic fluctuations. The timing of these phases has varied, even within the Mediterranean area itself, with differences between the north and south margins of the Mediterranean Sea. Much of the evidence cited points to the importance of understanding the type and scale of alterations and also their location and their relationship within the fluvial system, because of the effects on connectivity of runoff


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and sediment and the delivery downstream. This influences where impacts occur and how far changes are transmitted. These are, of course, mediated spatially by the natural morphology and by lithology. Much of the work and the data on rates of erosion has been based on plot studies (e.g. Boix-Fayos et al., 2005). Problems of upscaling are recognised by many researchers but detailed study of the nature of coupling, linkages, and connectivity inform the extent to which generalisations can be made from one part of the system to another. Many authors agree that what is needed now is a landscape approach but using information at a hierarchy of scales. This is already being used in practical management in Australia, following the important work of Ludwig and Tongway (2000), Tongway (2003). The importance of connectivity is also the premise for a EU funded project, RECONDES (, which is examining strategies for mitigating and restoring desertified land in the Mediterranean region through vegetation. Vegetation may reduce sediment removal and connectivity if located in hotspots of erosion and pathways of sediment delivery, e.g. on agricultural terraces. Much research in Europe in recent years has been stimulated by the perceived threat of desertification and much evidence has been advanced to demonstrate increasing problems of erosion and land degradation. However, some obverse trends are also apparent, at least if examined using remote sensing data and at large regional scales. Mulligan et al. (2004) challenge the idea of increased desertification and many case studies in that volume (Mazzoleni et al., 2004a,b) demonstrate increased vegetation cover. They also show, however, very differing patterns of fragmentation or homogenisation of land use and cover, with some differing arguments about the effects of this on connectedness. Connectedness of ecological or vegetation patches must be distinguished from pathways of connectivity of runoff and sediment. However, global warming scenarios of climate change and of land use responses tend to indicate an increased propensity to flash flooding and an increased drying that will make more areas marginal to dense vegetation growth. Boer and Puigdefabrigas (2005) have recently demonstrated the effects of the patterns of vegetation on erosion at the hillslope scale. The importance of riparian and channel vegetation in influencing erosion and transmission of sediment loads downstream is emerging but conflicting opinions and policies exist with regard to forest cover of floodplains. In the past, many countries have regarded riparian forest as causing logjams in floods and the

presence of forest on floodplains as decreasing the potential flood capacity. Now, many researchers are advocating the retention or increase of floodplain forests as increasing biodiversity and contributing positively to river dynamics and morphology, as well as possibly causing water retention in certain parts of valleys. This all raises questions for management policy and practices over the aims and goals of management and the landscape to be created or sustained. The inherent lack of equilibrium and the high variability in many Mediterranean systems, combined with the long and intense history of human impact, pose challenges to identifying how fluvial systems in the region should be managed now. Proposals of integrated watershed management may help but it will need detailed analysis of linkages and pathways within the system to understand impacts and feedbacks over various timescales. 7. Conclusions The Mediterranean environment is regarded as one of the most heavily impacted regions by humans. Most of the trends have been towards decreasing vegetation cover and increasing land degradation over time, though the length and timing of these phases and the nature of their impacts have varied. In the European Mediterranean some evidence points to a reversal of that trend now and for much increased retention of sediments within channel systems, with massive scales of adjustment by narrowing and incision apparent on many rivers in the last few decades. Some evidence also exists for recent increases in vegetation cover, partly from large-scale land abandonment. On the other hand, very large land clearances, land levelling, soil remodelling and highly industrialised farming are now occurring in parts of the European Mediterranean. Much direct destruction of fluvial landforms has also occurred, notably affecting badlands, alluvial fans and braided rivers. Traditional land practices, particularly construction of agricultural terraces on steep slopes, had the effect of reducing slope length and steepness and prevented erosion. New land practices or lack of terrace maintenance are reversing that trend and creating or opening up very long, smooth slopes. This geomorphic smoothing is leading to high potential for gullying and increasing the connectivity and sediment delivery again. A key to understanding spatial variability and patterns of human impact is to analyse the presence of fluvial barriers, buffers and discontinuities in the landscape, and to examine the specific pathways and linkages at various temporal and

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