Meteorological and Land Use Controls on Past and Present
Total Page：16
File Type：pdf, Size：1020Kb
HYDROLOGICAL PROCESSES Hydrol. Process. 17, 3287–3305 (2003) Published online in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/hyp.1387 Meteorological and land use controls on past and present hydro-geomorphic processes in the pre-alpine environment: an integrated lake–catchment study at the Petit Lac d’Annecy, France G. C. Foster,1*J.A.Dearing,1 R. T. Jones,1 D. S. Crook,1 D. J. Siddle,1 A. M. Harvey,1 P. A. James,1 P. G. Appleby,2 R. Thompson,3 J. Nicholson3 and J.-L. Loizeau4 1 Department of Geography, University of Liverpool, Roxby Building, Liverpool L69 7ZT, UK 2 Department of Applied Mathematics, University of Liverpool, Liverpool, UK 3 Department of Geology and Geophysics, University of Edinburgh, Kings Building, Edinburgh, UK 4 Centre d’´etudes en Sciences Naturelles de l’Environnement, Institut F.-A. Forel, Universit´edeGen`eve, 1290 Versoix, Switzerland Abstract: A wide range of environmental records is integrated in order to reconstruct the mechanisms of flooding and sediment transport within the 170 km2 Petit Lac catchment, Annecy, France, over time scales of 101 to 102 years. These records include sequential lake sediment trap samples and cores, floodplain stratigraphies, dated landform assemblages, hydro-meteorological records, and documented histories of river channel and land-use change. Mineral magnetic measurements are used as the basis for classifying catchment sediment sources and tracing sediment movements through time. Records of magnetic susceptibility for monthly sediment trap samples (1998–99) track seasonal discharge, peaking in winter and spring. Magnetic records in lake sediment cores are compared against and tuned to precipitation records to provide dated proxy records for past discharge spanning sub-annual to decadal time scales back to 1826. Calculated sediment accumulation rates in lake sediment cores are used as proxies for time-averaged catchment sediment load. Analysis of the results reveals that climate and land-use controls on the hydrological and sediment system are complex and vary according to the time scale of observation. In general, cycles of agricultural expansion and deforestation appear to have been the major cause of shifts in the sediment system through the late Holocene. Deforestation in the 18th century may have caused a number of high-magnitude flood and erosion events. As the time scale of observation becomes shorter, changes in climate and hydro-meteorological conditions become progressively more important. Since the mid-19th century, smoothed records of discharge roughly follow annual precipitation; this is in contrast to sediment load, which follows the trend of declining land-use pressures. Episodic erosion events during this recent period seem to be linked to geomorphic evidence for slope instability in the montane and sub-alpine zones, triggered by intense summer rainfall. At the annual scale, changes in seasonal rainfall become paramount in determining sediment movement to downstream locations. The study demonstrates that the connections between forcings and responses span a four-dimensional array of temporal and spatial scales, with strong evidence for dominantly nonlinear forcing–response mechanisms. Copyright 2003 John Wiley & Sons, Ltd. KEY WORDS Lac d’Annecy; lake sediments; mineral magnetism; erosion; flooding; human impact; climate INTRODUCTION A review of projected climatic change over the next century in the western French Alps anticipates increased mean temperatures and precipitation (Gyalistras et al., 1998). This leads to a number of general questions about hydro-geomorphic mechanisms in the densely populated pre-alpine landscape. How are soil erosion and * Correspondence to: G. C. Foster, Department of Geography, University of Liverpool, Roxby Building, Liverpool L69 7ZT, UK. E-mail: [email protected] Copyright 2003 John Wiley & Sons, Ltd. 3288 G. C. FOSTER ET AL. flooding related to different combinations of land use and climate? Can land use be managed effectively in order to reduce the worst effects of flooding? To what extent do past and present interactions between climate and human activities condition future impacts? These questions form the basis of research at Lac d’Annecy, Haute-Savoie, designed to further our understanding of the synergies between climate and human activities through analyses of documentary and sedimentary archives. Previous research at Annecy has demonstrated the use of lake sediment magnetism in reconstructing records of soil erosion (Higgitt et al., 1991) and flooding (Thorndycraft et al., 1998). Using an extended lake–catchment approach, Dearing et al. (2001) proposed that the lake sediment signal could be disaggregated in terms of shifting contributions of sediment derived from magnetically distinct catchment sources that correspond broadly with altitudinal soil zonations. Following a sharp erosional response to forest clearance at ¾1000 cal. years BP, the role of low–mid-altitude surface soils and high montane soils as sediment sources show divergent trends (Dearing et al., 2001), with the contribution from the latter gradually increasing up to the present day. This may simply reflect the enhanced storage of surface soil on the floodplain after 2000–1000 cal. years BP. Alternatively, it may imply that while the low–mid-altitude soil–vegetation systems showed some degree of stabilization over subsequent centuries, the high montane zone progressively destabilized. We reassess the significance of these proxies over 101 –102 year time scales in the light of new information from sediment trap sampling, marginal lake sediments, field geomorphological studies, and hydro-meteorological data, supplemented with documentary records of flooding, land use and channel changes. The aims of this work are: (1) to establish calibrations between contemporary and recent magnetic signals in trapped and lake inflow sediments; (2) to assess the extent to which geomorphic proxies can be extrapolated to the long-term record; and (3) to make a first attempt at explaining the significance of hydro-meteorological and land-use forcings on flooding and erosion over the past few centuries. SITE DESCRIPTION Lac d’Annecy lies in the Haute-Savoie region (lat. 45°480N; long. 6°80E), situated at an altitude of ¾447 m within the pre-alpine region of the French Alps. The lake comprises two sub-basins, the Grand and Petit Lacs, surrounded by a catchment with mountain summits up to 2351 m (Figure 1). The total lake surface area of 26Ð5km2 is fed by a total catchment area of 251 km2. The majority of the catchment (170Ð4km2 drains into the Petit Lac (area 6Ð25 km2), giving a lake basin : catchment ratio of ¾27. Three major streams flow into the Petit Lac: the Eau Morte, Ire and Bornette (Figure 1). The Eau Morte is the largest in terms of length, sub-catchment area and the proportion of total discharge to the lake. There is some evidence to suggest that the St Ruph and Tamie´ sub-catchments (Figure 1) have periodically drained eastwards away from the present Eau Morte course at Faverges, but during at least the historical period the Eau Morte has drained most of the valley and the majority of the farmed land in the catchment, flowing for ¾10 km through a wide floodplain (area ¾4km2) in the lower reaches. There is clear evidence for canalization of the Eau Morte and lower reaches of the Ire dating from at least the last century, and for artificial drainage of the lower floodplain linked to 20th century intensive agriculture. Natural reaches of the Eau Morte display a meandering channel geometry, in contrast to the Ire, Bornette, and other upland tributaries that are confined gravel-bed mountain torrents. River discharge records between 1975 and 1998 show that 65% of maximum annual floods occur in the period November–March, often linked to snowmelt, whereas June–August are characteristically months with low flow, except during short-duration and high-intensity storms. Documentary records since 1570 show a similar pattern, with 57% of floods recorded in the months November–May. The geology of the Petit Lac catchment is dominated by limestones and marls of different facies, ranging from Jurassic to Tertiary age (Higgitt, 1985), which to a large extent provide a structural control on terrain and geomorphic processes. A range of erosional processes operate on the steep valley sides and montane slopes. Slopes above ¾1500 m (high montane zone) and above 2000 m (sub-alpine zone) are dominated by mass movement processes, particularly debris flows. The debris flows are commonly coupled with the ephemeral Copyright 2003 John Wiley & Sons, Ltd. Hydrol. Process. 17, 3287–3305 (2003) PAST AND PRESENT HYDRO-GEOMORPHIC PROCESSES AT LAC D’ANNECY, FRANCE 3289 200 km 400 TALLOIRES 1600 Le Nant d'Oy N Le Petit Lac FRANCE 800 ▲ La Tournette STUDY ● LA13 (2351m) 1200 AREA Montmin PL8 ● PL1 ● MONTMIN PLC13 ● Lathuille Ire ● ST Grand Lac Doussard Bornette DOUSSARD Laudon 600 Petit Lac au M E or te km Chevaline ● 0 5 Eau PLC1 Ire Morte Giez Commune boundary 800 FAVERGES Faverges LATHUILLE 1200 GIEZ Seythenex St. Ruph R 1200 . a 1600 u CHEVALINE T 1200 a m i SEYTHENEX e 1600 Pointe de la Sambuy (2198m) ▲ 1600 0 5 km Col de Tamié Figure 1. Petit Lac d’Annecy: main inflows, sampling sites, commune boundaries, place names and topography. Upper inset shows location in France; lower inset shows the connected Grand and Petit Lacs and the location of contemporary sediment trap sampler (ST) Copyright 2003 John Wiley & Sons, Ltd. Hydrol. Process. 17, 3287–3305 (2003) 3290 G. C. FOSTER ET AL. headwater stream network, which typically attains drainage densities >10 km km2, resulting in the efficient delivery of eroded material to the fluvial system. Extensive talus and scree deposits have accumulated below bedrock cliffs, and commonly these provide the source material for mass movement processes. The valley sides of mountain torrents, such as the St Ruph and Ire (Figure 1) between ¾1000 and 1500 m (montane zone), are typically >30° and are highly susceptible to gullying.