Réf. Schulte & al. 2009 - A

Référence bibliographique complète

SCHULTE, L., VEIT, H., BURJACHS, F. JULIÀ, R. 2009. Lütschine fan delta response to climate variability and land use in the Bernese Alps during the last 2400 years. Geomorphology, Vol. 108, 107–121.

Abstract: Despite important progress in Holocene palaeoclimate and palaeoenvironmental studies in the Alps during recent years, the knowledge of Alpine river dynamics and their chronology is still limited. To address the influence of external factors, such as climatic variability and land use, on aggradation and floods, we focused our research on the Late Holocene sedimentary and geomorphologic history of the Lütschine fan delta. Morphological mapping, sedimentology, geochemistry, palynology, historical maps, building inventories and 14C-dating techniques provided valuable data for the reconstruction of palaeofloods, wetland environments, and anthropogenic impact. The sedimentological findings showed a general agreement between geochemical and pollen changes and δ14C anomalies from 2400 to 850 yr cal. BP. Furthermore, the coarse-grained layers, deposited during palaeofloods about 400 yr cal. BC and 100, 700, 1100, 1550, and 1830 AD, correlate with positive radiocarbon anomalies, suggesting that aggradation in the Lütschine fan delta during the focused period was triggered by solar forcing. According to our pollen data and the traced correlations, the Lütschine river floods occurred during cold and wet periods. The return interval of recorded flooding events during the last 2400 years varies between 300 and 600 years. With regard to the last millennium, anthropogenic impacts, such as woodland clearance for grazing and agriculture and river diversion, have changed the sedimentary and geomorphic pattern on the Lütschine fan delta, reducing wetland environments.

Mots-clés

Fluvial environments - Late Holocene - Climate variability - δ14C anomalies - Swiss Alps

 

Organismes / Contact

Department of Physical Geography and Laboratory of Landscape Research, University of Barcelona, Carrer de Montalegre 6, E-08001 Barcelona, Spain - L. Schulte: schulte@ub.edu
• Institute of Geography, University of Bern, Hallerstrasse 12, CH-3012 Bern, Switzerland
• ICREA at the Department of Prehistory, University Rovira i Virgili, Plaça Imperial Tarraco 1, E-43005 Tarragona, Spain
• Institute of Earth Science “Jaume Almera”, CSIC, C/Lluis Sole i Sabaris s/n., E-08028 Barcelona, Spain

 

(1) - Paramètre(s) atmosphérique(s) modifié(s)

(2) - Elément(s) du milieu impacté(s)

(3) - Type(s) d'aléa impacté(s)

(3) - Sous-type(s) d'aléa

Solar forcing

 

Palaeofloods

 

 

Pays / Zone

Massif / Secteur

Site(s) d'étude

Exposition

Altitude

Période(s) d'observation

Swiss Alps

Bernese Oberland

Lütschine river (tributary of the River Aare)

 

601–564 m a.s.l.

Last 2400 years

 

(1) - Modifications des paramètres atmosphériques

Reconstitutions

 

Observations

 

Modélisations

 

Hypothèses

 

 

Informations complémentaires (données utilisées, méthode, scénarios, etc.)

 

 

(2) - Effets du changement climatique sur le milieu naturel

Reconstitutions

 

Observations

 

Modélisations

 

Hypothèses

 

 

Sensibilité du milieu à des paramètres climatiques

Informations complémentaires (données utilisées, méthode, scénarios, etc.)

 

 

 

(3) - Effets du changement climatique sur l'aléa

Reconstitutions

Results

Geochronology

(…) Based on the age–depth models, five major aggradation phases, interrupted by peat horizons, were identified for the period from 2200 yr BP to the present for the Lütschine fan delta:
1. From 2200 to 1980 yr BP, aggradation rate of 2.9 mm/yr
2. From 1980 to 1850 yr BP, aggradation rate of 3.7 mm/yr
3. From 1850 to 1650 yr BP, aggradation rate of 1.3 mm/yr
4. From 1650 to 1160 yr BP, aggradation rate of 1.4 mm/yr
5. From 1160 yr BP to 1860 AD, aggradation rate of 0.5 mm/yr (…)

Sedimentology

The sedimentology of the fan delta deposits, studied in five well-exposed profiles and in nine cores, records several sedimentation phases of riverbed sediments, organic-rich silt layers and peat horizons from 4420 ± 40 yr BP to the present. (…) [see details in the study]

Geochemistry

To understand the relationship between the sediment source area in the catchment and major lithologies in the Lütschine profile IN-2, 46 sediment samples were selected for analysis, representing about one analysis every 5 to 10 cm. The altitudinal distribution of the bedrock lithologies in the catchment could be a key factor to explain the lithological changes in the Lutchine fan. The geochemical stratigraphy of the major inorganic elements is illustrated. The results show high concentrations in SiO2, Al2O3, Fe2O3, and CaO, which is in accordance with the aluminosilicate and limestone bedrock mineralogy of the Lütschine catchment. Iron, Ti, K and Mg occur abundantly in the phyllosilicate minerals present in crystalline and metamorphic rocks (biotite and chlorite); K is derived from phyllosilicates (biotite and muscovite) and feldspars. (…) The given geochemical evolution indicates the existence of two important sedimentary sources, one related to calcium contribution and the second to siliciclastics. During peat deposit formation the Ca content shows the lowest values, probably due to acid waters of these environments, while fine sediments rich in phyllosilicates were deposited as well. It is possible to differentiate at least four major aggradation phases for the Lütschine fan according to grain-size distribution and factor 1. The general tendency towards lower CaO content in the fan delta sediments records a progressive contribution of sediment yield of areas with crystalline rock.

Geomorphology

(…)The identification of palaeochannels, their migration, abandonment and control by man thus requires a complex multi-proxy approach From the combined geomorphological survey based on fieldwork and aerial photographs the authors studied the spatial configuration of the Lütschine channel branches shown on historical maps published between 1496 and 1900. The location of historical buildings and archeological sites on the fan delta indicates minimum ages of channel abandonment, whereas the ages of bridges — e.g. the Gsteig bridge dating from 1738 yr AD — provide post quem dates of active river channels. According to the reconstruction, the Lütschine river splitted into four branches during historical times. For reconstruction the authors considered only maps resulting from in situ surveys. (…) From these data, along with the geochronology and sedimentology of transect cores, the authors conclude that the nearest western channel to the IN-2 key section shifted westwards during Roman times and again during the early Middle Ages, and was finally abandoned between 1750 and 1796. In the eastern part of the Lütschine fan delta the two closest channels were abandoned between 1750 and 1796 and between 1796 and 1811. The increasing distance between the active channels and the IN-2 and IN-10 key sections during the last two millenniums in the western part of fan delta and during the last four centuries in the eastern part implies two things: first, the local sedimentation rate decreased in the upper unit, and second, aggradation occurred without any important erosive phases.

Pollen results

In order to provide evidence of forest clearance, the relationship between the non-arboreal pollen and arboreal pollen content (NAP/AP) obtained from the IN-2 key profile were plotted versus age. The percentages of mesic trees (Fagus, deciduous Quercus, Acer, Castanea, Tilia, Carpinus, Corylus) were considered to highlight the climate signal. These pollen data record vegetation dynamics over the last 2200 ±70 yr BP and can be subdivided into two major zones. The lower zone shows a dense tree vegetation cover (AP = 80%), with mesic trees peaking up to 20%. From the beginning of the sequence, human activity is documented by wood clearances for local agriculture (Cerealia, Linum, Cannabaceae) and grazing (Plantago, Asphodelus). However, the vegetal ecosystem is still sustainable, as is demonstrated by the arboreal pollen recovery. The upper pollen zone extends from 1160 ± 20 yr BP until the present and records a continuous regional deforestation (AP = 20%) as consequence of human impact since the 12th century. Human impact and, partly, anthropogenic river diversion (Vischer, 1989 and Vischer, 2003) — both related to the economic expansion of the Monastery of Interlaken — changed the depositional conditions, reducing wetland environments. The upper sample records the present reforestation trend, with AP percentages reaching 70%.

Discussion

1. Geomorphic evolution of the Lütschine fan delta

According to previous literature (Hantke and Wagner, 2005), deglaciation of the Aare and Lütschine glaciers in the study area (Bödeli area) occurred around 18,000 yr cal BP (Unterseen/Interlaken stage). Afterwards, the Lütschine glacier evolved separately from the Aare glacier system, and subsequent fan delta aggradation by the Lütschine and Lombach river began.

The Holocene deposits of the Lütschine River only represent a thin portion of the 550-m-thick Lütschine fan delta (Kellerhals and Haefeli, 1985). As reported by Bodmer et al. (1976) based on pollen data from a core located at the distal part of the fan delta at the hospital of Interlaken, the Alleröd was traced at a depth of 30 m. Nevertheless, the stratigraphic Late Pleistocene/Holocene boundary is not clearly defined due to the absence of deep cores and radiometric dating (Kellerhals and Haefeli, 1985). Hinderer (2001), who calculated Late Quaternary erosional denudation for the major river systems of the Alps, assumes a decrease of the mean accumulation rate of the Lütschine river from the Late Glacial (6.0 mm yr− 1) to the Holocene (2.5 mm yr− 1).

Our Late Holocene data agree with these assumptions, and two radiocarbon dates of 5071 ± 204 and 4742 ± 116 yr cal. BP were obtained at a depth of 15 m in the IN-11 core. Furthermore, the detailed chronological model of the IN-2 profile of the last 2200 yr BP suggest that the local aggradation rate was more or less constant until Roman times (about 2.9–3.7 mm yr− 1), when a moderate reduction occurred until 1078 ± 92 yr cal. BP (1.3–1.4 mm yr− 1). A subsequent drastic drop in sedimentation rate (0.5 mm yr− 1) was found and attributed to Lütschine channel migration, along with extensive land management.

The debate regarding the pattern and migration of the Lütschine channels during historical times was opened by Vischer (1989) who postulated, based on studies of historical documents, that the Lütschine river was subject to anthropogenic control since the 13th century, predating the corrections of the Kander (1714), Linth (1811), Aare (1866–1873) and Melchaa (1880) rivers. The author concluded that between the 13th century and the definitive Lütschine correction in the early 19th century, the Lütschine river was diverted to the eastern margin of the fan delta. However, Vischer (2003) admits that the Lütschine broke through the levee system during floods, returning to its natural ramified fan delta drainage network.

Our multi-proxy approach suggests that successful river management was probably limited to the upper reach of the fan delta from the apex to the Gsteig Bridge built in 1738. The historical maps from 1538, 1734 and 1750 show four and three ramified river channels, detected also in the aerial photograph survey, field mapping, and cores, which were at least periodically active during this period. Extensive flooding on the Lütschine fan delta has been recorded even in the 19th, 20th and 21st centuries. During the last flood in August 2005, the Lütschine broke through the levees near Wilderswil and followed the old palaeochannel traced by our survey in 2001 (Schulte et al., 2003 and Schulte et al., 2004).

However, sedimentological and pollen data from the IN-2 key section record important local environmental changes in the floodplain. From 1078 ± 92 yr cal. BP onwards, the sedimentation pattern changed from fluvial and paludal deposits to predominately fine sandy beds producing a noticeable decrease in the aggradation rate. With regard to vegetation changes, a general drop in arboreal pollen percentages suggests progressive deforestation. These data raise the question: to what extent did land use relate to the foundation and prosperity of the Monastery of Interlaken in 1133 AD (Affolter et al., 1990) and how did it influence the geomorphic history of the Lütschine fan delta?

The study of the historical maps (e.g. Topographische Karte der Schweiz by Dufour, 1845–1865) and aerial photographs shows positive lakeshore changes and telescope delta formation from 1578 AD to present days at the eastern margin of the Lütschine fan delta. These deposition processes could be related to improved river management confining the eastern Lütschine channels.

2. Environmental reconstruction of the Lütschine fan delta

The Late Holocene depositional and vegetation history of the Lütschine fan delta records important environmental changes. The sedimentology shows the alternation of wetland environments with large lateral extent and floodplain deposits. Radiocarbon dates obtained from the bottom and top of two peat horizons in the IN-2 key section shows that the minerotrophic fen rose by 14 cm from 1790 ± 73 to 1758 ± 59 yr cal. BP and by 12 cm from 1638 ± 76 to 1531 ± 82 yr cal. BP, respectively. Two main periods of fan building can be differentiated as described below.

2.1. Natural-dominated fan delta environments

The earlier period extends from 2176 ± 167 yr cal. BP to 1078 ± 92 yr cal. BP and records a woodland environment affected by climate changes and occasionally by anthropogenic clearances. Despite the anthropogenic influence, the independent variables of CaO and mesic pollen percentages draw a similar pattern.

Geochemistry and mesic tree percentages show at least four aggradation phases. The first phase ranges from approximately 2176 ±167 yr cal. BP to 1935 ± 51 yr cal. BP and starts with high arboreal pollen percentages and low calcium content, followed by a progressive increase in calcium, herb pollen and mesic trees. The second phase starts with a change towards more frequent flooding episodes with gravel aggradation extends from around 1935 ± 51 yr cal. BP until 1790 ± 73 yr cal. BP. This flood episode correlates with the beginning of a period of increased debris-flow events (1850–1750 yr cal. BP) recorded by turbiditic layers in lake sediments from Lago di Braies (Dolomite Alps, N. Italy) according to Irmler et al. (2006). Similar grain-size peaks were reported from the lake sediments of the Unterer Landschitzsee (Lower Tauern, Austria) and were interpreted to be the result of an extended snow cover during a cold and humid climatic phase (Schmidt et al., 2002). This cold phase has also been found in stalagmite records from the central (Mangini et al., 2005) and southeastern Alps (Frisia et al., 2005) and from high lake-level reconstruction in the Jura massive and Swiss Plateau (lake-level Phase 4 according to Magny, 2004). This second phase ends with double peat layer that extends from 1790 ± 73 until 1531 ± 82 yr cal. BP showing a similar pattern of a progressive increase in calcium and mesic tree percentages. Despite of both environmental variables increasing until an interpolated age of 1400 yr cal. BP, we use the peat beds as lithological criterium to define the phase.

Subsequently, drops in mesic trees and calcium occurred and a gravel layer was deposited. This new phase of flooding from around 1400 to 1250 yr cal. BP could correspond to the cold and wet period defined by increased debris flows in the Dolomite Alps (1300–1200 yr cal. BP; Irmler et al., 2006), lower δ18O‰ from speleothems in the central and southeastern Alps (1300 yr cal. BP; Mangini et al., 2005 and Frisia et al., 2005) and high lake levels (1300–1100 yr cal. BP; Magny, 2004). This third phase ended with the deposition of a new peat layer dated at 1078 ± 92 yr cal. BP which also records an increase in mesic taxa and calcium content.

The fourth phase started around 1078 ± 92 yr cal. BP at the top of the peat layer when the organic-rich levels with a large lateral continuity disappeared from this area of the Lütschine fan delta, probably as a consequence of land management.

2.2. Fan-delta environments under preponderant land use

The second main fan-delta period corresponds to the aggradation during the last millennium and is characterised by a strong human impact responsible for the drop of arboreal pollen percentages and mesic trees, as well as the absence of organic-rich layers. This deforestation activity for the purpose of grazing and farming in the Lütschine catchment is related to the foundation of new settlements and churches in the Bernese Oberland during the 11th century, the economic prosperity of the Interlaken Monastery (1133 AD), and local mining from the Middle Ages (first documents from 1465 AD; personal communication of the Archeological Survey of the Canton of Berne) to 1805 AD.

First documents of the altitudinal Alpine grazing system date from 994 AD, although the general extent of the Almwirtschaft (Alpine transhumance) developed from the 12th century on, causing woodland clearance in two directions: from the valley floor, where settlements are located, uphill (our pollen data) and from the natural alpine meadows, above the timberline, downhill (Lotter et al., 2000). In addition, during the medieval clearance period agriculture reached its maximum extent on the valley floor, while during the 17th century farming only served subsistence purposes and rural economy was dominantly based on grazing and cheese production (Affolter et al., 1990).

Human impact on woodland was also related to lead, zinc and iron mining in the Lauterbrunnen valley (western Lütschine catchment). An interruption of iron mining in 1715 was caused by wood shortage due to excessive deforestation (Affolter et al., 1990).

Tourism in the Bernese Oberland started during the 18th century, related to the Naturalists (Romantisism), and during the first half of the 19th century visitors were attracted by Alpinism (first ascent of the Jungfrau in 1811), spas, and local festivals. Interlaken played a leading role in the development of the Swiss tourism. During the second half of the 19th century the construction of lodges, palace hotels and the railway caused important construction activity on the Lütschine fan delta near Interlaken, Wilderswil and Matten. Furthermore, agricultural activities on the Lütschine fan delta were forced by the construction of drainage systems in the 1860s under the management of the Prussian Count von Burtales (personal communication of the Archeological Survey of the Canton of Berne).

Among the three gravel layers of the IN-2 key section occurring during the last millennium, only one was radiocarbon dated. The middle fluvial bed at a depth of 40 cm was dated to 404 ± 89 yr cal. BP.

The age of the lower layer was linearly interpolated between the 1078 ± 93 yr cal. BP peat horizon and the 404 ± 89 yr cal. BP date and provide an age of around 850 yr cal. BP.

We suggest an age around 1830 AD for the upper coarse-grained bed based on the presence of artifacts as pottery fragments dated at end of the 18th c. below and drainage pipes from 1860 AD above. Furthermore, historical documents (Vischer, 1989) indicate severe flooding and aggradation up to 1 m at the village Wilderswil during 1831.

The 404 ± 89 yr cal. BP detrital layer matches with the 1547–1610 yr AD cool pulses of the Little Ice Age recorded by tree-ring, documentary, lake-level and glacier proxy data from the Swiss and Austrian Alps (Pfister, 1999, Pfister, 2004, Magny, 2004, Casty et al., 2005, Wilson et al., 2005 and Holzhauser et al., 2005). In the Dolomitic Alps, debris-flow frequency increased substantially from 1500 to 1625 AD (Irmler et al., 2006). In addition, Schmidt et al. (2002) report an increase in grain-size in lake sediments coeval with an expansion of Pinus mugo types, interpreted as increased denudation processes during cold climate conditions, for the Unterer Landschitzsee (Lower Tauern). With regard to river dynamics in the Swiss Alps, based on documentary data, Pfister (1999) found a higher flood frequency of the Alpine Rhine, Rhone and Reuss rivers between 1550 and 1580 AD, principally related to cold and wet years with increased spring snow melt as recorded in 1566 AD. Prolonged ice and snow cover during the late 16th century is documented also by palynological studies of Hagelseewli lake, located at 2339 m a.s.l. and a distance of 12 km from the Lütschine fan delta (Lotter et al., 2000).

Concerning glacier fluctuations and their possible contribution to fan delta aggradation during the last two millennia, data are available only for the Lower Grindelwald glacier. From the six major advance periods reported from this glacier, only three (1088–1137 AD, 1547 AD, and 1820–1860 AD according to Holzhauser et al., 2005) coincide with principal flood events, recorded around 1100 yr AD, at 1546 ± 89 AD and around 1830 yr AD in the Lütschine fan delta. However, the uncertainty of the radiocarbon dates is no help in rendering these correlations more precise, and the conclusions are weakly supported.

3. Geomorphic processes and the sediment supply mechanism

The key section IN-2 records the distal Lütschine fan delta evolution during the last 2400 yrs. It shows the dominance of siliciclastic sediments and the inverse relation between siliciclastic and carbonate-rich sediments. In addition, a positive correlation exists between mesic tree pollen and CaO contents showing three of the four phases described in Section 5.2.1. This geochemical pattern of sediment supply results from the contribution of two principal source areas: crystalline rocks that extend up to 4158 m a.s.l. at the summit area of the Aare Massif and carbonate rocks with maximum elevations between 2343 and 2928 m a.s.l..

Two main textures provide evidence of geomorphic processes involved in the fan-delta evolution: coarse-grained sediments and fine-grained sediments including peat and organic horizons. The coarse-grained sediments correspond to fluvial deposits interpreted as periods of mayor sediment supply from areas where siliciclastic sediments with minor carbonate contribution predominate. Organic-rich and fine-grained deposits record more distal environments with a relative major contribution of carbonate sediments, probably from nearby slope erosion.

According to the mesic arboreal pollen percentages the fluvial contribution occurred in the transition periods between maximum and minimum percentages. We suggest that during mild-humid periods enhanced paraglacial slope adjustment after deglaciation — affecting lateral moraines, kame terraces, slopes, rock faces etc. by rock fall, landslides, debris flows, and rill erosion (Hinderer, 2001 and Schrott et al., 2002) — provides significant amounts of sediment as has occurred during recent decades. For example, on May 30th 2005 landslides affected the east of the Eiger involving a total of 600,000 m3 of sediments. On July 13th 2006 rock falls moved a total of 2,000,000 m3. Both gravitational processes occurred in response to the rapid reduction of ice volume of the Unterer Grindelwald glacier since the end of the Little Ice Age (Keusen et al., 2007 and Zumbühl et al., 2008). Towards cold-drier phases of the Late Holocene, sediments provided by these paraglacial processes were transported downstream jointly with unconsolidated deposits from slopes, talus cones, valley floor, etc.

Nevertheless, these unconsolidated deposits contain abundant silica rocks mixed with minor limestone. Therefore, erosion phases produced during rainstorms and floods contribute to the erratic variability found in the chemical composition of coarse fluvial beds.

Assuming a climate signature in the sedimentary aggradation processes in the Lütschine fan delta, the residual δ14C, which is a function of the intensity of solar irradiation (Stuiver et al., 1997 and Lean, 2005), should indicate a similar pattern. The two main geochemical and pollen parameters are compared with the radiocarbon anomalies, showing a general agreement between earlier three phases found in the aggradation and the δ14C anomalies; coarse-grained layers deposited during palaeofloods about 100, 700, 1100, 1550 and 1830 yr cal AD correlate with positive radiocarbon anomalies.

Concluding remarks

From their findings the authors draw the following conclusions:

1) The Lütschine fan delta records show several phases of fluvial beds, organic-rich silty layers and peat horizons from 5071 +/− 204 yr cal. BP to the present.

2) The fluvial archives of the Lütschine fan record palaeoclimatic variability from 2400 to 1000 yr cal BP. For the last millennium, the anthropogenic impact changed the depositional conditions, reducing wetland environments. Moreover, this impact masks the climate signal of the pollen and geochemistry proxies.

3) The correlation between the major flooding events and δ14C anomalies suggest that aggradation of the Lütschine fan delta during the focused period was triggered by solar forcing. The return interval of recorded flood events of the last 2400 years varies between 300 and 600 years.

4) According to the sedimentary record and traced correlations, floods occurred in the Lütschine River catchment during cold and wet periods.

5) The calibration of the natural proxies with documentary and instrumental data should be improved by increasing the number of case studies of Alpine catchments to check our fluvial response model.

Observations

 

Modélisations

 

Hypothèses

 

 

Paramètre de l'aléa

Sensibilité des paramètres de l'aléa à des paramètres climatiques

Informations complémentaires (données utilisées, méthode, scénarios, etc.)

Flood frequency

 

This research uses a multi-proxy approach to gain a deeper understanding of the fluvial and geo-ecological dynamics of the Lütschine fan delta (601–564 m a.s.l.) over the last 2400 years. The choice of the Lütschine fan delta as a case study has several reasons: first, the availability of high-resolution flood records from the Swiss Alps and the Rhine catchment for the last centuries based on documentary and instrumental data (Pfister, 1999, Wanner et al., 2004 and Glaser et al., 2005), along with high-resolution glacier proxies from the Lower Grindelwald Glacier (Lütschine catchment) for the Late Holocene (Holzhauser et al., 2005); second, the spatial coexistence of wetland, alluvial and fluvial environments on a low-gradient fan delta in connection with a lake, generating a high-resolution sedimentary record; third, the specific spatial lithological settings of crystalline and carbonated sedimentary rocks in the Lütschine catchment; and, finally, the existence of exceptional exposures and their accessibility during the construction of the “Theme Park of Mysteries” near the town of Interlaken. The study aims to integrate data from the Lütschine fan delta with other case studies of fluvial and glacial environments in the Alps.

The Late Holocene depositional history of the Lütschine fan delta was reconstructed using a multi-proxy approach that combines different methods from several disciplines such as sedimentology, geomorphology, palynology, and geochronology. The first group of analyses focuses on sedimentary changes in several key sections; the second group covers spatial changes in the fan morphology; the third group investigates the variability of the local vegetation; and the last group deals with the use of proxy-data sets from the alluvial fan as an integrated fluvial palaeoclimate record. [See details in the study]

Eight AMS radiocarbon dates (Angström Laboratory, University of Uppsala) and 22 conventional radiocarbon dates (Radiocarbon Laboratory, Physics Institute, University of Berne) of peat material (11), wood (6), charcoal (1) and bulk sediment (12) provide the basis for age–depth modelling of the Lütschine alluvial fan.

 

(4) - Remarques générales

Gently sloping surfaces appropriate for settlement are scarce in high mountains ranges, and they are commonly related to distal parts of alluvial fans that are prone to flood hazards. Sedimentology, geomorphology and pollen analysis from local sites provide valuable data that can contribute to understanding the influence of external factors, such as climatic variability and land uses, on aggradation and flooding processes on an alluvial fan.

Alluvial fans and fan deltas in alpine areas constitute major zones of fluvial sedimentation. Such sedimentation is intermittent in time and space, and sediments may be removed by subsequent reworking phases. Despite the resulting discontinuity of sediment records, such sediments provide accurate data about terrestrial environmental change, including changes in the hydrological regime, sediment supply, and land use (Schrott et al., 2003). The Alps, which form a border between the humid mid-latitudes under Atlantic influence, to the north, and the Mediterranean subtropical zone, to the south, are especially sensitive to events of extreme precipitation and disastrous floods, and to changes in the circulation of the atmosphere at the global scale (Wanner et al., 2004). Natural palaeofloods are known to have resulted from excessive rainfall intensity and frequency, snow melt, glacier melt, precipitation combined with frozen soils, and other phenomena. Land use, as well, can substantially modify mountain ecosystems and the dynamics of river systems (Glaser et al., 2005).

Despite important progress in palaeoclimate and palaeo-environmental studies in the European Alps during recent years, precise data on Holocene Alpine river dynamics and their chronology are still limited. Sedimentological and geomorphic studies on alluvial fans have been undertaken in the High Tauern (Veit, 1988), Inn Valley (Patzelt, 1994), Ötztal Alps (Geitner, 1999), Retic Alps (Burga et al., 1997), Allgäu Alps (Jerz et al., 2000) and the Bavarian Alps (Schrott et al., 2002 and Schrott et al., 2003). However, regional correlation of Alpine fluvial chronostratigraphies reveals a heterogeneous pattern of geomorphic processes (Schulte et al., 2003 and Schulte et al., 2004).

However, temporal resolution of alluvial fan and fan delta deposits and fluvial terraces is often low due to their architecture (aggradation of detritic sediments, cut and fill). Furthermore, the number of radiocarbon dates obtained is low. Alpine fan deltas generally provide high resolution records that show differentiated sedimentary sequences containing organic-rich beds and peat horizonts (Burga et al., 1997), but as geological sections are almost entirely absent, correlation between the different cores requires precise dating of the sequences.

In addition to these sedimentary and methodological problems, we must take into account that asynchronisms in fluvial processes between different catchments are also a result of these processes being driven by different forces (connection to glaciers, extent of periglacial area, snow melt, vegetation changes, land use, etc.). Glaser (2001) demonstrated for larger catchments of central Europe that periods of increased flood occurrence in the Elbe, Weser, Rhine, Pegnitz and Danube catchments during the last 700 years did not always coincide with regard to meteorological extreme events. Glaser (2001) explained the differences in flooding between these catchments by relating them to spatially limited rainfall events of local or micro-regional extent. Such differences in regional flood response have also been shown by the work of Gees (1997) that focused on extreme events in Swiss river basins over the period from 1800 to 1994.

 

(5) - Syntèses et préconisations

 

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Archäologischer Dienst des Kantons Bern, 1982. Archäologisches Hinweisinventar für den Kanton Bern. Archäologischer Dienst des Kantons Bern, Bern (updates).

Blumer, W., 1950. Die Schweizer Karten von Gilg Tschudi und Gerhard Mercator. Geographica Helvetica 5, 190–193.

Bodmer, R., Matter, A., Scheller, E., Sturm, M., 1976. Geologische, seismische und pollenanalytische Untersuchungen im Bödeli bei Interlaken. Mitteilungen der Naturforschenden Gesellschaft in Bern. Neue Folge, 30, pp. 51–62.

Bundesamt für Landestopographie, 2004. Atlas der Schweiz Version 2.0 (electronic data). Bundesamt für Landestopographie, Berne.

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