Meteorological and Land Use Controls on Past and Present

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’études en Sciences Naturelles de l’Environnement, Institut F.-A. Forel, UniversitédeGenève, 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]

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 .

1600 u CHEVALINE T 1200 a m

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. The spatial distribution of gullies appears to reflect slope destabilization due to past phases of woodland clearance, which in the modern landscape determine the position of avalanche tracks and chutes. Modern land use in the Petit Lac catchment is strongly controlled by altitude and terrain. Coniferous woodland on steep montane slopes at mid-altitudes merges with high montane grassland at 1400–1600 m. Cultivated land and improved pasture exist in several montane valleys and are separated from the intensively cultivated Eau Morte valley floor by steep lower slopes covered by mixed (beech and fir) woodland. Soil types vary from thin skeletal rendzinas in sub-alpine zones and on steep slopes to fully developed brown earths on lower and flatter terrain, where there may be variable thicknesses of glacial tills. Some valley floor soils are poorly drained and exhibit evidence of gleying. A key method employed in earlier studies, and this, has been the use of mineral magnetic properties (Thompson and Oldfield, 1986; Walden et al., 1999) to define sediment sources and to infer erosional processes in sediment records. Limestones that are very poor in primary magnetic minerals dominate the geological units in the catchment. The non-diamagnetic properties of the limestones are almost completely dominated by goethite, and the marls contain iron-bearing paramagnetic minerals and goethite (Hu, 1997). Rissian and Wurmian¨ glaciers (Evin et al., 1994) deposited at least two types of till: one contains abundant schist erratics derived from the inner Alps and is dominated by paramagnetic minerals with low concentrations of primary ferrimagnetic minerals (Hu, 1997); the other is composed of limestones and marls considered to be of local origin (Beck et al., 1996; Hu, 1997; Hu et al., 2001). Early magnetic and Mossbauer¨ studies showed that many of the surface soils in the well-drained parts of the catchment are relatively rich in fine-grained secondary ferrimagnetic minerals (SFMs) produced by pedogenic enhancement mechanisms (Dearing, 1979; Longworth et al., 1979; Dearing et al., 1996). The main factors that control SFM concentrations appear to be drainage and microclimate, with lithology exerting a major local influence only where glacial tills contain primary magnetite and possibly haematite in schist erratics. Soil magnetic maps (Dearing et al., 2001) for the Petit Lac catchment show four main groups of soil (Table I), used below as the basis for sediment source ascription.

MATERIALS AND METHODS Lake sediment traps and cores Two sediment traps were suspended in the connected Grand Lac (Figure 1) at water depths of 20 and 46 m. Both sediment traps comprise six cylindrical acrylic collecting tubes housed within PVC frames. Each tube has a diameter of 11 cm and length of 80 cm, giving an aspect ratio of 7Ð3, in accordance with the recommendations given by Bloesch and Burns (1980) for small lakes, and providing a total collecting surface of 570 cm2. The traps were emptied on 18 visits between April 1998 and December 1999, with trap periods ranging between 20 and 41 days. The samples collected (mass range 0Ð72–7Ð55 g) were dried for a range of analyses, which included mineral magnetism by the methods outlined below. The results shown here are from the 46 m trap only (results from both traps are highly repeatable), except for paramagnetic data that derive from sediment in both traps combined together for each time period. Earlier studies (Dearing et al., 2001, Noel¨ et al., 2001) were based on the analysis of a single 8Ð14 m sediment sequence (Kullenberg core LA13), collected from the central plain (Figure 1) of the Petit Lac (water depth 55 m), comprising ﬁne silts and clays with a moderately high calcium carbonate content. More recent sampling retrieved 18 ¾1 m cores from a variety of locations in the Petit Lac basin. The present study focuses on the upper 1 m section of LA13 and the 1 m long core PL8 taken from 28 m water depth ¾300 m north of the main Eau Morte delta.

Copyright  2003 John Wiley & Sons, Ltd. Hydrol. Process. 17, 3287–3305 (2003) PAST AND PRESENT HYDRO-GEOMORPHIC PROCESSES AT LAC D’ANNECY, FRANCE 3291

Table I. Magnetic classiﬁcation of soils and potential sediment sources (after Dearing et al. (2001))

Soil Magnetic mineralogya Magnetic parameters Location group

1 Fine-grained SP and SSD High LF,SOFT%,FD and Free-draining soils on lower slopes secondary ferrimagnetic FD%; low PARA % <10% and ﬂatter areas of montane minerals; low concentrations of valleys paramagnetic minerals 2 Primary SSD and MD High values of SIRM, Two discrete zones of schisty till ferrimagnetic minerals; relatively HIRM, SIRM/LF and lack derived from outside the large antiferromagnetic of covariance with FD; catchment south of the village of component; minor contribution low values of PARA % DoussardandintheTamie´ valley by SP secondary ferrimagnetic minerals; small contribution by paramagnetic minerals

3 Signiﬁcant antiferromagnetic high HIRMH%; high PARA % Ire and Bornette valleys and the ‘haematite’ component; >50% high montane and sub-alpine negligible SP secondary zones of the Montmin and Tamie´ ferrimagnetic minerals valleys.

4 Signiﬁcant antiferromagnetic High HIRMG%; high Unconsolidated and unweathered ‘goethite’ component; negligible PARA % >50% substrates in high montane and SP secondary ferrimagnetic sub-alpine zones of the Montmin minerals and Ire valleys a SP: superparamagnetic; SSD: stable single domain; MD: multidomain.

All sediment samples were freeze-dried prior to physical, geochemical and magnetic analyses. Dry bulk density was calculated as the dry mass per unit volume for each lake sediment sample; particle size measurements were made on ¾1 g of sediment using a Coulter LS130 laser diffraction granulometer, following removal of organic matter by hydrogen peroxide; and total elemental concentrations were measured on ¾0Ð5g samples using a Metorex X-ray fluorescence system. Magnetic analyses included standard measurements (Thompson and Oldfield, 1986; Walden et al., 1999) of low- and high-frequency magnetic susceptibility (LF and HF, permitting the calculation of frequency-dependent magnetic susceptibility, FD and FD%), susceptibility of anhysteretic remanence ARM, isothermal remanence (IRM) at back fields of 20, 100, 300 and 1000 mT and in-field magnetization measured in a vibrating sample magnetometer (VSM). Full details are presented in Dearing et al. (2001).

Geomorphological studies Floodplain surveys were carried out in autumn 1999 in order to identify the nature and sequence of fluvial deposition on the Eau Morte valley floor. Between 2 and 3 m of sediments are exposed along dyked reaches of the river, but where possible the sampling was extended to levels below the modern channel bed by gouge coring. At some sites, the uppermost channel fills were morphologically and sedimentologically described in order to determine the nature of channel changes prior to modern dyking operations. Lichen-based slope geomorphic studies were carried out in autumn 1998 in order to investigate the timing of regional sub-alpine debris-flows (above ¾1500 m) in the Petit Lac catchment and the Chaine des Aravis, some ¾10 km to the east of the lake. Aerial photographs indicate the presence of debris-flow tracks covering ¾10 km2 of the catchment, primarily on La Tournette and at Sambuy (Figure 1). Unfortunately, the application of lichenometry within the catchment area is severely restricted due to the unfavourable substrate for the growth of the lichen species Rhizocarpon geographicum. However, preliminary surveys indicated that the lichen-rich fossil depositional landforms of the neighbouring Chaine des Aravis were similar in nature to those within the

Copyright  2003 John Wiley & Sons, Ltd. Hydrol. Process. 17, 3287–3305 (2003) 3292 G. C. FOSTER ET AL. catchment, and were likely to represent a similar geomorphic history. This study focuses on the findings from two debris-flow sites, at the Alp de Fier (map OT 3531, 0917 2099) and Col de l’Arpettaz (map OT 3531, 0917 2096), representative of slope processes operating between 1500 and 2100 m on hillslope and mountain- front landscape units respectively. On-cone landforms were mapped, and debris-flow lobes were distinguished from alluvial or fallen deposits. Thalli diameters for R. geographicum can be used to estimate the minimum age of deposition if the local growth rate is known. Measurements taken at local graveyards are limited to the period after around 1970, and suggest a diametric growth rate of 0Ð7 mm year1 (Foster, 2001). Beyond this time scale, the likelihood for reduced lichen growth rates has been approximated, providing age estimates of ¾90 years and ¾120 years for individuals of 40 mm and 50 mm diameter respectively. These growth rates correspond broadly with the lower range of values reported from sites in the UK (0Ð6–1Ð0 mm year1; Milne, 1983; Topham, 1977; Harvey et al., 1984) and Swiss Alps (0Ð8mmyear1; Proctor, 1983). Tentative oldest- age estimates for debris-flows were made by calibrating the mean a-axis diameter of the largest population of lichens on each landform surface to the local growth curve. Where present in sufficient numbers, at least ten largest lichen were measured at each site using calipers with an accuracy of ¾0Ð1 mm. Stratigraphic and soil profile observations were made to supplement the lichen surveys.

RESULTS AND INTERPRETATION

Channel changes, historical flooding and floodplain development The most dramatic channel changes on the lower floodplain since before 1730 have included at least four shifts of the Ire torrent (Figure 2). The position of the 18th century Ire is marked by linear channel fills on the floodplain surface that appear to reflect natural avulsions prior to 1730. Written accounts of the flooding of Verthier (Figure 2) would suggest that the 1906 course was established before 1825 and remained until around 1918, after which the modern dyke was constructed. Historical records spanning 1725–45 suggest that the frequency of flooding of the main channels peaked on five occasions, ca 1750, 1800, 1850, 1875, and 1900 (Crook et al., 2002). The floodplain sediment sequence comprises lower, inorganic clay and an upper silt unit, commonly capped and often truncated by coarse channel fills to a depth of ¾2 m. Close to the lake (e.g. PLC13, Figure 1) the lower clay unit overlies at least 1 m of organic-rich sediments, ranging from ¾20 cm thick, silty soils to coarse sandy–organic flood couplets. The presence of a consistent suite of coarse horizons and palaeochannels along the length of the upper floodplain provides strong evidence for recent flooding and river channel changes. The channels contain imbricated cobbles set within sandy gravels, and indicate a final stage of channel adjustment and rapid floodplain accretion before major dyking operations in the early 20th century. Summary results (Table II) of channel studies at site PLC1 (Figure 1) show that a reduction of width : depth ratios between the lower and middle stages was accompanied by a shift to assymetrical channel geometry, indicative of meandering. This signals a reduction in gradients, perhaps in response to reduced sediment calibre (Table II), sediment load or stream power. The upper channel signals a return to a more unstable system, in which braiding occurred possibly in response to increased sediment supply. The upper channel (e.g. PLC1) contains anthropogenic artefacts (e.g. brick fragments and a bed spring), linking it to the channel depicted on the 1906 cadastral map. Two radiocarbon measurements of macrofossils and charcoal in a flood horizon ¾30 cm below the base of the channels at site PLC1 give modern dates with maximal ages of ¾280 cal. years BP. Therefore, the channel fills reflect rapid aggradation within the recent historical period, perhaps post-1700. Two radiocarbon measurements of wood fragments in organic-rich gravels at the contact with underlying clays and overlying silts at PLC13 (Figures 1 and 2) give dates of 2351–2746 and 2212–2713 cal. years BP (2) respectively, suggesting that overbank deposition started during Iron Age times and that no major coarse flood deposits were laid down between ¾2500 and ¾300 BP.

Copyright  2003 John Wiley & Sons, Ltd. Hydrol. Process. 17, 3287–3305 (2003) PAST AND PRESENT HYDRO-GEOMORPHIC PROCESSES AT LAC D’ANNECY, FRANCE 3293

PL8

Le Petit Lac

PL1 PLC13

1730

Ire E a u

1810

Verthier

1906

Pre-1730 0Metres 300

Figure 2. Historically mapped channel changes for the rivers Ire and Eau Morte on the lower ﬂoodplain since before 1730, with location of lake sediment coring positions PL1 and PL8

Table II. Morphological and sedimentology of Eau Morte palaeochannels

Channel Dimensions (m) D50 Regime W; D; ratio (a; b; c (cm))a

Upper 14; 0.8; 17 na Sinuous to multi-threaded Middle 5; 1.5; 3 8.7; 5.8; 3.8 Meandering Lower 11; 1; 11 10.3; 7.7; 5.2 Sinuous Modern 15; na; na 11.9; 8.6; 4.5 Dyked and straightened

a Median of measurements on 50 cobbles.

Sub-alpine slope instability At the Alp de Fier, a debris cone has accumulated at the base of a 0Ð1km2 talus-mantled catchment. Fossil depositional landforms include three sets of lobes and a distal alluvial fan (Figure 3a), indicating that the development of the cone has involved both debris ﬂow and ﬂuvial reworking processes. Recent reworking

Figure 3. Debris cone geomorphology at (a) the Alp de Fier (perspective view) and (b) Col de l’Arpettaz (plan view) has resulted in the development of a modern trench and distal gravel fan. Lichen sizes measured on the fossil landforms fall into three groups, the earliest of which are group 1 lobes and the fossil fan, which have the largest thalli, ¾40–42 mm, suggesting stabilization by ca 1900–05. A second lichen group has thalli of 24–27 mm and represents the group 2 lobes and distal levee´ boulders, indicating debris-ﬂow activity ca 1945–55. Group 3 lobes represent the most recent fossil landforms (thalli 11–12 mm), dated to ca 1980.

Copyright  2003 John Wiley & Sons, Ltd. Hydrol. Process. 17, 3287–3305 (2003) PAST AND PRESENT HYDRO-GEOMORPHIC PROCESSES AT LAC D’ANNECY, FRANCE 3295

The fossil debris-flow landforms at the distal end of the large (¾600 m axial length) debris cone at Col de l’Arpettaz are set into a ‘high’ alluvial fan surface (Figure 3b). The soils on the flow deposits are limited to ¾5 cm organic horizons, in contrast with those developed on the fan surface, which display a 30 cm deep, weakly aggregated profile. The different stages of soil development indicate that the fan represents a much earlier phase of slope instability than the later debris-flow deposits. The earliest debris-flow landforms occur as large bouldery lobes, upon which largest lichen thalli of 50–52 mm indicate deposition ca 1870–80. A second flow resulted in the formation of crested and partially slumped levees,´ upon which mean thalli diameters of 34–36 mm suggest a timing of ca 1920–30. A third flow appears to have deposited within- and near-channel lobes, upon which lichen have grown to 24–26 mm, indicating debris-flow activity in the period ca 1948–55.

Contemporary sediment traps Comparison of selected magnetic and physical properties of sediments for each sampling period with summed monthly and daily discharge (Figure 4a) shows that bulk LF varies cyclically from minima

(a) 300 25 ) 1 − ) s 1

Sum per interval Daily mean

− 250 3 s 20 3 200 15 150 10 100

50 5 Sum discharge (m 0 0 Daily mean discharge (m

(b) 30 1.5 1 1 −

χ χ − LF FD kg

20 1.0 3 SOFT % 3 m

8 8 − − 10 SOFT % 10 10 0.5

χ χ 0 0.0

10 % 10

HARD % PARA

χ 5 0

0 JJASONDJFMAMJJAS Figure 4. Magnetic properties of sediment trap samples and river discharge (June 1998 to September 1999): (a) summed river discharge for each period of trap sampling and mean daily discharge; (b) measurement values of LF, FD and SOFT% for sediments deposited in each trap period; (c) measurement values of PARA %, HARDH% and HARDG% in sediments deposited each trap period

(2 ð 108 m3 kg1) in summer to a maximum (24 ð 108 m3 kg1 in winter and spring (Figure 4b). SOFT% is inversely related to LF, ranging from ¾30% in summer and autumn to ¾25% in winter (Figure 4b). The presence of pedogenic SFMs is implied by peak FD during autumn and winter months. Relatively high proportions of paramagnetic material (PARA % >10%) occur over winter and spring, whilst the inputs from ‘haematite’ (HARDH%) and ‘goethite’-rich (HARDG%) sediment increase simultaneously to reach late spring maxima (Figure 4c). The seasonal cyclicity of magnetism in the trap samples indicates that LF may provide a sensitive proxy for mean monthly stream discharge (QM) that could be applied to lake sediment cores. The relationship between LF and QM is strengthened when LF is expressed in terms of the non-CaCO3 mass, showing that dilution by diamagnetic CaCO3 (40–80% by mass) can be discounted as a control factor. Before using LF as a proxy for QM, a physical basis must be identified. A range of interrelated hydrodynamic factors can control LF, including variations of suspended sediment calibre, sources, and delivery rates and processes (Dearing, 1999). Positive associations between QM, median particle size, and LF indicate that the magnetic cycles may partly reflect discharge-controlled variations of particle size. However, in samples of PL8 sediment, LF exhibits bimodal peaks in clay and coarse silt fractions, but it is low in the fine silt fraction (Foster, 2001). Comparison between samples with low (‘summer’) and high (‘winter’) LF values also reveals that magnetic enhancement is focused in coarse silts. Whether the coarse silt fractions are derived from the delivery of enhanced surface soil (soil group 1) or from small pockets of till (soil group 2) has yet to be ascertained (see Table I). However, as the degree of coupling between slope, floodplain and channel systems increases during floods, it may be expected that both soil groups 1 and 2 become increasingly entrained in the sediment load. The seasonality of HARD properties and PARA % indicates that primary minerals derived from high montane and sub-alpine soils (soil group 3) and parent material (soil group 4) form an increased proportion of sediment through the winter/spring snowmelt (see Table I). In part, this may reflect drainage expansion into upland gullies and ephemeral streams with increasing precipitation, and the coupling of high-altitude slopes to low- order streams afforded by snowmelt runoff. A further source for soil groups 3 and 4 magnetic signals is cohesive floodplain clays that have been exposed by dyking operations to form the physical boundary of main channels, and which are readily eroded by slab failure during floods. The timing of enhanced inputs of non-ferrimagnetic sediments in the trap sediments, therefore, appears to be consistent with the hydrodynamic effects of winter processes. The sequence of SOFT% in the trap samples is consistent with its use to infer the delivery of soil group 1 (Table I) during intense and highly erosive summer/autumn rainstorms (e.g. Thorndycraft et al., 1998). However, SOFT% in the traps falls below values found in low–mid-altitude topsoils; rather, they are typical of values in soil groups 3 and 4 (Dearing et al., 2001). Data from flood layers at floodplain section PLC1 (Figure 1) reveal order of magnitude enhancement (¾80%) in pedogenic clays relative to coarser fractions (Foster, 2001). Therefore, it appears likely that peak SOFT% values in lake sediments can be used to indicate the delivery of group 1 soils (Table I) to the lake sediments, but that values <¾30% reflect inputs from additional sediment sources. Further evidence for the delivery of group 1 soils is provided by the association of peak FD with individual storms (Figure 4). The two FD peaks follow the onset of autumn and winter runoff, probably reflecting entrainment into the fluvial system by rilling and overbank inundation respectively. The absence of FD peaks for subsequent runoff events of comparable magnitude indicates the seasonal exhaustion of available topsoil sources. It may be anticipated that, in lake sediment, FD might reflect the interplay between the timing and erosional impact of individual storm events on surface soil.

Recent lake sediments: inter-annual process-response Core PL8 comprises calcareous, olive brown, clayey silts that include six organic-rich dark layers below 33 cm. Dry bulk density (DBD) falls in the lower sediments, but this trend is reversed above 33 cm. Values for LF and SOFT are inversely related to DBD, rising through six apparent cycles (C1–C6, Figure 5) in the lower sediments and declining above 33 cm. Earlier studies of deep-water sediment cores suggest that

Copyright  2003 John Wiley & Sons, Ltd. Hydrol. Process. 17, 3287–3305 (2003) PAST AND PRESENT HYDRO-GEOMORPHIC PROCESSES AT LAC D’ANNECY, FRANCE 3297 s of ferrimagnetic Clay % Clay 30 40 -1 -1 -1 -1 Fe mg g Fe Ca mg g 20 25 Ca mg g Fe mg g Fe 30 45 60 15 -3 g cm 0.9 1.2 Dry density bulk ked C1–C6 and S1–S4 respectively. Horizontal dotted lines mark position % PARA χ 10 20 30 % % G H cal properties (total Ca and Fe) of marginal core PL8 with apparent cycles % H % G of dark organic-rich layers HARD HARD 0.0 HARD HARD 51016 -1.5 1.5 -1 kg 2 S4 S3 S2 S1 Am 5 SOFT % SOFT 25 50 SOFT % 25 20 30 45 SOFT 10 C6 -1 C4 C1 C2 C3 C5 kg % 3 % FD LF m FD χ χ -8 χ 10 LF χ and SOFT) and peaks in the proportions of ferrimagnetic minerals (SOFT%) mar 6912 04812 LF

10 20 30 40 50 60 70 80 Depth (cm) Depth concentrations ( Figure 5. Selected magnetic, dry bulk density, clay percentage and geochemi

Copyright  2003 John Wiley & Sons, Ltd. Hydrol. Process. 17, 3287–3305 (2003) 3298 G. C. FOSTER ET AL. the detrital magnetic signal is overprinted with magnetite of bacterial origins (Higgitt, 1985; Thorndycraft et al., 1998). However, detailed studies of core PL8 and PL1 (Figure 2) provide little evidence for bacterial or post-depositional modification; rather, they indicate a dominantly detrital origin for the shallow water inflow sediments (Foster, 2001). Peak values of FD%(>8%), indicative of the delivery of lowland enhanced soils (group 1; Table I), coincide with the organic-rich layers. Values of SOFT% covary with SOFT, though oscillations between 20 and 30% in the lower sediments oppose the concentration cycles. Four SOFT% peaks (S1–S4, Figure 5) occur between 23 and 7 cm. Peak values of PARA % and HARDH%, together indicative of high montane, sub-alpine and unweathered parent material (groups 3 and 4; Table I), occur towards the base of the sequence; maxima coincide with minimum concentrations of the apparent LF and SOFT cycles. Total Ca and Fe covary with ferrimagnetic concentrations and values of clay% throughout the majority of the core, supporting a dominantly detrital origin for the sediments. The apparent ferrimagnetic cycles C1–C6 below 33 cm directly oppose the variations of clay%, but above 33 cm the clay% covaries with LF and SOFT. Direct dating of the core by 210Pb and 137Cs gives imprecise results due to extreme dilution of atmo- spherically derived radionuclides by high and variable sediment accumulation rates (SARs). An independent chronology is therefore based on the extrapolation of a 137Cs marker horizon from dated master cores in the central plain of the lake, using a LF-based core correlation scheme (Jones et al., 2003). The correlation indicates that the sediments post-date peak 137Cs levels associated with 1963 nuclear-bomb testing, and suggest that the 75 cm level corresponds with a date of 1968 š 5. Thus, PL8 is estimated to span the period from 1968 š 5 to 1999, giving a mean SAR of 2Ð5cmyear1. Monitored records of the monthly mean of daily discharge QM for the Eau Morte are available for part of this time scale, 1975–99, corresponding to the upper ¾55 cm of the core, and records of monthly precipitation PM at Annecy are available for most of the period back to 1873. To test the idea that LF may be used as a discharge proxy, as described above for the sediment traps, the QM and PM records were smoothed using a 12-point moving average to account for the time-integrated effects of mixing and bioturbation within the sediment record. The resulting filtered time series for discharge (Q12) and precipitation (P12) were compared with the sediment LF record (Figure 6a). Visual curve matching with peaks and troughs in the LF trace above 56Ð5 cm (cf. 1975 D 55 cm), with minimal curve stretching, 2 gives R values of 0Ð76 and 0Ð64 for correlations with Q12 and P12 respectively. This suggests that LF cycles C1–C6 (78–33 cm) were deposited between 1973 and 1978, straddling the onset of river discharge monitoring in 1975. Investigations of the hydro-meteorological significance of cycles C1–C6 are therefore limited to comparisons with the longer P time series. Hydro-meteorologically derived age estimates for the base of the core lie within the dating errors of, and thus broadly support, the radionuclide chronology. A comparison between the dated LF record and precipitation data for the period covering the section of sediment containing cycles C1–C6 shows a good correspondence with P12 and an even stronger match to a six-point smoothed curve (P6; Figure 6b). This strongly suggests that LF cycles C1–C6 represent annual depositional features for the period 1973 to 1978. To test the repeatability of these associations, the LF trace from the Ire inflow core PL1 (Figure 1) was curve-matched to the P12 record (not shown). Again, there is a high degree of similarity between the sediment and instrumental record, resulting in a linear correlation 2 (R D 0Ð80). These findings provide strong evidence for hydrological forcing of the recent sediment LF signal on annual to sub-annual time scales and confirm the use of LF as a sedimentary proxy for monthly discharge and precipitation. Using the hydro-meteorologically tuned chronology for PL8, SARs are calculated for 29 time periods between 1973 and 1999 (Figure 7a). The mean SAR is 3 g cm2 year1 with a declining trend towards the present and episodes of high SARs reaching 16 g cm2 year1 between 1973 and 1978, and during 1989. The maximum SAR appears to coincide with peak precipitation levels in 1977, but high SARs over the period 1973–76 and in 1989 correspond with periods of relatively low precipitation. This suggests that runoff is not always the dominant control on catchment sediment loads. Rather, peak sediment load is more frequently controlled by sediment supply and availability than by precipitation and discharge.

Copyright  2003 John Wiley & Sons, Ltd. Hydrol. Process. 17, 3287–3305 (2003) PAST AND PRESENT HYDRO-GEOMORPHIC PROCESSES AT LAC D’ANNECY, FRANCE 3299

(a) 1.0 12

-1 0.8 kg

3 10 m -8 0.6 10 LF

χ 8 Normalised Q, P χ 0.4 LF 12 point smoothed Q 6 12 point smoothed P 0.2 1976 1978 1980 1982 1984 1986 1988 1990 1992 1994 1996 1998

(b)

160 6 point smoothed P 12 12 point smoothed P 140 χ LF 120 -1

10 kg 3

100 m -8 P mm 80 10 8 LF χ 60

40 Cycle 1 Cycle 2 Cycle 3 Cycle 4 Cycle 5 Cycle 6 6 1973 1974 1975 1976 1977 1978 1979 Year AD

Figure 6. Calibration of LF records in core PL8 with discharge and precipitation data: (a) curve-matched LF and the normalized 12-point smoothed monthly discharge Q and monthly precipitation P over the period 1975–99; data normalized by dividing monthly P and Q values by the maximum P and Q value over the 25 year time period; (b) curve-matched LF and the six-point and 12-point smoothed monthly precipitation P over the period 1973–78

Recent lake sediments: sub-annual process-response

During the period of cycles C1–C6 (1973–78) the presence of covarying high values of SAR, FD% and PARA %, especially in the winter–spring seasons of 1973, 1975, 1976–77 and 1978 (Figure 7b and c), emphasize the importance of snowmelt processes for the coupling of upland and lowland sediment sources with the fluvial network. During this period, the covarying trends of SAR and PARA % (Figure 7b) suggest that upland-derived material (soil groups 3 and 4; Table I) provides the dominant sediment source and exerts the strongest control on sediment loads. From 1975 (the date of the oldest discharge records), peak values of FD% (Figure 7c) show a tendency to rise with increasing flood magnitude, suggesting that the entrainment of lowland soil (group 1; Table I) takes place progressively as lowland valleys flood and as sediment in temporary channel storage sites becomes mobilized. Declining SARs during the late 1970s (Figure 7a) coincide with a major change in the seasonal distribution of precipitation, from summer/autumn peaks during 1973–77 to a winter/spring maximum in 1978, suggesting that SARs are controlled by the seasonality of rainfall, and particularly the occurrence of highly erosive, intensive rainstorms. Furthermore, daily discharge data reveal that each of the years 1975, 1976 and 1977

(a) 12 point smoothed P SAR 160 16 140

12 -1

120 yr -2

100 8 P mm

80 4 SAR g cm 60 0 19751980 1985 1990 1995 1999

(b) SAR χ % 16 PARA 30 HARDH %

-1 12 25 yr % -2 % H 20

8 PARA χ HARD 15 SAR g cm 4 10

1973 1974 1975 1976 1977 1978 1979 (c) 40000 χ Q FD % 12 30000 -1 8

20000 % Q l s FD χ 4 10000

0 0 1973 1974 1975 1976 1977 1978 1979 Year AD Figure 7. Hydro-meteorologically tuned and dated PL8 records and monthly precipitation and discharge data: (a) PL8 SARs and 12-point smoothed monthly precipitation data P in the period 1973–99; (b) calculated SARs and magnetic parameters (PARA %, HARDH%) indicative of soil groups 3 and 4 (see Table I) in the period 1973–79; (c) PL8 FD% record and daily mean discharge Q for cycles C1–C6 in the period 1973–79. Note the different time scales as shown by dotted lines between (a) and (b) witnessed one high-magnitude (mean Q>104 ls1) summer/autumn storm event. Therefore, relatively high sediment contributions from both upland and lowland sources, and the high SAR values, reﬂect the supply and availability of sediment following intensive rainstorms—often several months prior to deposition in the lake. The seasonal lag between peak precipitation in summer/autumn and the winter/spring sediment delivery by snowmelt presumably represents the temporary storage of eroded material at slope base and near-channel locations. The magnitude of snowmelt is far less important in determining sediment loads than the degree of slope–channel coupling and the size of potential sediment stores.

Copyright  2003 John Wiley & Sons, Ltd. Hydrol. Process. 17, 3287–3305 (2003) PAST AND PRESENT HYDRO-GEOMORPHIC PROCESSES AT LAC D’ANNECY, FRANCE 3301

Lake sediments: the past two centuries

The successful calibration of magnetic data with hydro-meteorological data over seasonal and annual time scales is extended to the past two centuries in the upper 1 m of LA13 (Figure 1). The main features in the 5 year smoothed record of precipitation at Geneva (¾50 km north of Annecy) extending back to 1826 (Figure 8a) correspond reasonably well with the LF record in the central core LA13 (Figure 8b), suggesting that LF may also be used as a first-order proxy of discharge and precipitation over longer time scales. The accuracy and precision of the tuned chronology for LA13 is confirmed by 210Pb and 137Cs measurements and a pollen-based event chronology (Jones et al., in preparation), and is used to calculate mean SARs (Figure 8c). The trend in the discharge proxy record (Figure 8b) shows gradually rising peak and minimum values from 1826 to the present. In contrast, the sediment yield record (Figure 8c) shows a declining trend of minimum values from ¾0Ð63 to 0Ð1gcm2 year1 with five or six periods of high SARs ranging from ¾1Ð25–0Ð6g cm2 year1. There is only a partial correspondence between the peak discharge and peak sedimentation rate, suggesting that annual river discharge records are more closely related to annual meteorological conditions, and annual precipitation in particular. Therefore, it appears that sediment supply to the lake over the longer time scale is controlled to a large extent by factors other than annual and seasonal meteorology.

(a) 1600

1400 -1

1200 P mm yr 1000

(b) 16

-1 14 kg 3

m 12 -8 10

LF 10 χ

-1 1.25 yr 2 1.00 0.75 0.50 SAR g cm 0.25

1825 1850 1875 1900 1925 1950 1975 2000 Date (AD)

Figure 8. LA13 records (upper ¾1 m) compared with Geneva precipitation records 1826–1995: (a) 5 year smoothed annual Geneva precipitation P;(b)LF record from upper LA13 curve-matched to (a); (c) SAR in upper LA13 calculated using curve-matched chronology

DISCUSSION: HYDROLOGICAL AND SEDIMENT DYNAMICS Over millennial time scales, long sediment records in the Petit Lac may be compared with pollen diagrams, regional land-use syntheses and climate reconstructions to demonstrate that shifts in the sediment supply have been forced largely by land-use changes, especially upland deforestation in the period 800–1100 AD (Dearing et al., 2001). Decadal, annual and sub-annual proxy records, presented here for discharge and sediment load within the past 200 years, offer the opportunity to extend the analysis of climate or land use as forcing factors at different time scales. For recent centuries, documentary and census data for the catchment provide information about potential land-use forcings on the hydrological cycle in the catchment (Figure 9). Human population reached a peak in the middle 19th century before declining in the ﬁrst half of the 20th century as

(a) 8000

7000

6000 Population 5000

(b) 1500 Faverges Giez Doussard Montmin Seythenex Chevaline 1000 Lathuille

500

Cultivated land (ha) Cultivated 0

3000 Forest cover (ha) cover Forest 1000

16 (d) 1.50 SAR χ

LF -1

-1 1.25 14 kg yr 3

-2 1.00 m

12 -8 0.75 10

0.50 LF

10 χ SAR g cm 0.25 8 1825 1850 1875 1900 1925 1950 1975 2000 Date (AD) Figure 9. Documentary records and hydrological proxies for the Petit Lac catchment 1825–1999: (a) total human population (up to 1975); (b) cultivated land in seven communes (see Figure 1); (c) forest cover for the total catchment and upland communes; (d) discharge (LF) and sediment yield (SAR) proxies from LA13

Copyright  2003 John Wiley & Sons, Ltd. Hydrol. Process. 17, 3287–3305 (2003) PAST AND PRESENT HYDRO-GEOMORPHIC PROCESSES AT LAC D’ANNECY, FRANCE 3303 part of a regional trend of rural depopulation (Figure 9a). This trend was reversed from the 1950s onwards as town communities (e.g. Faverges) and tourism expanded. Over the same period, the amount of cultivated land declined (Figure 9b) by more than 50% in some of the catchment’s communes, though a small rise in two of the curves for the lowland communes (F and D) describes a local resurgence in intensive cultivation practices after 1945. Forest cover (Figure 9c) increased by ¾175% over the catchment as a whole and by ¾350% in the upland communes, with some deforestation registered in upland communes (e.g. Montmin) in the early and late 19th century. Comparison of these records with the proxy discharge record (Figure 9d) shows that ‘higher than expected’ peaks in the LF record ca 1890 and during the 1960s might argue for amplified flood levels caused by pre-1900 deforestation and 20th century dyking, urbanization and intensive cultivation. But otherwise, annual flood magnitudes appear to be highly tuned to annual precipitation: large changes in land use have not triggered systematic shifts in rainfall–runoff relationships. The timing of widespread floodplain instability and destructive flooding within the past 300 years is not sufficiently constrained but is most likely to pre-date the mid 19th century. Phases of deforestation along the main river valleys were partly driven by developing proto-industrial activities from the 17th century onwards, reaching levels as high as 10% of all forest per year in the late 18th and early 19th centuries. This suggests that the important zone of runoff generation, at high montane and sub-alpine altitudes above the tree-line (¾1500 m) where precipitation levels are highest, has not been affected by land-use transformations over the past few centuries, though this is unlikely to have remained the case over millennial time scales. In contrast, declining sediment load since 1825, a trend that is also observed at a finer time scale since 1974 (Figure 7a), suggests that the reductions in agricultural intensity and improved forest conservation have affected baseline sediment supplies. Short- term peaks in sediment load since 1825 (Figure 9d) coincide with short periods of deforestation before 1850, ca 1880 and post-war cultivation practices 1945–70, but the timing of sediment pulses since ca 1880 is also closely linked to the widespread activation of debris flows every 20–40 years. The evidence suggests that, although underlying trends of sediment supply are driven by land use, this does not exclude short-term sediment responses to changes in climatic seasonality, or seasonal to inter-annual changes of land use and agricultural practices driven by technological or socio-economic factors. At the decadal time scale, climate and land use may interact on the sediment system in complex ways that are difficult to resolve. At the annual time scale, sediment load is at a maximum at the time of snowmelt, and in recent decades it is clear that the magnitude is greatly amplified by the erosive effects of rainfall in the preceding summer/autumn months. This lag-time represents the period between activation of largely upland slope erosion processes delivering unweathered substrate and the onset of efficient slope–channel coupling and sediment transport mechanisms during snowmelt. Additional surface soil is incorporated into the sediment load during snowmelt from sources close to montane and lowland channels, presumably by overbank flooding. Less common are discrete sediment pulses, usually dominated by topsoil sources, transported during intense storms outside the snowmelt period. Winter snowfall and rainfall are therefore second-order factors in controlling the magnitude of the sediment pulse. Summer storms may be effective in triggering rill processes, but they are only rarely effective at delivering sediment from surfaces to channels. However, their importance in determining sediment supplies at all time scales longer than the season should not be understated. Intense summer rainfall probably gives rise to similar frequency–magnitude relationships for two independent sets of processes within short and medium time scales: upland mass movements and lowland soil erosion. The effects of climatic and land-use forcings on discharge and sediment load are clearly related 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. As the time scale of observation becomes shorter, changes in climate and hydro-meteorological conditions become progressively more important. At the annual scale, changes in seasonal rainfall become paramount in determining sediment movement to downstream locations. We may certainly hypothesize that land-use changes have conditioned and, in the present situation, amplified later responses in the sediment system. This particularly applies to the early deforestation of the montane zone and cultivation in Iron Age and early medieval times (Dearing et al., 2001) that caused the rapid aggradation of overbank deposits on the valley floor after ¾2500 cal. years BP.It

Copyright  2003 John Wiley & Sons, Ltd. Hydrol. Process. 17, 3287–3305 (2003) 3304 G. C. FOSTER ET AL. also applies in the 18th and 19th centuries, when dramatic phases of deforestation led to rapid increases in sediment supply and floodplain accretion, channel changes and destructive flooding. These events may have triggered the transgression of sediment thresholds on steep slopes, which in turn increased their sensitivity to mass movement failure during extreme rainfall events in the 19th and 20th centuries. In a warmer and wetter climate, we might expect mean annual flood levels to follow roughly the late winter and spring precipitation. But careful management of the montane and sub-alpine zones seems critical for avoiding flashy hydrological responses, especially in view of the importance of erosive summer rainfall in determining to a large extent the annual sediment supply. These findings are beginning to clarify the connections between forcings and responses across a four- dimensional array of temporal and spatial scales. Not only are the controls on discharge and sediment delivery a consequence of time scale, it is also apparent that soil–vegetation–hydrological systems in diverse altitudinal zones, within the same catchment, have significantly different degrees of resilience to combinations of climate and human activities. The mix of forcings and scales means that some hydro-geomorphic responses are clearly direct, broadly linear and exhibit negligible time lags; other less obvious forcing–response relationships involve long-term and threshold-dependent nonlinear changes. Synergistic interactions between climate, human activities and hydro-geomorphic response in the Annecy catchment are therefore complex. An improved understanding of the relationships between environmental forcings and responses in this and other landscapes requires the adoption of the methodological framework used in this study, in which all relevant spatial and temporal scales are included and integrated.

ACKNOWLEDGEMENTS The research was funded by an NERC studentship to GF (4/97/158ES) and the Leverhulme Trust (F/25/BQ). We thank Fernand Berthier, Achim Brauer, Jacques Louis de Beaulieu, HerveNo´ el,¨ Frank Oldﬁeld and Elisabeth Verges` and others in the CLIMASILAC group for logistical help and discussions; we also thank Jack Hannam and Amy Clarke for ﬁeldwork assistance. The radiocarbon dates were provided by Jean-Luc Michelot, Universite´ de Paris-Sud.

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