Geomorphic map of the Malnant drainage, highlighting avalanche paths, mass movement, areas of afforestation, and the alluvial fan that has developed where the Malnant joins the upper Fier River. Transect A – A V is shown as a geologic cross-section in Fig. 6. 

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... response to the hillslope instability and associ- ated problems for valley residents, the French govern- ments purchased the upper 15% of the watershed between the years of 1883 and 1903 and began a program of reforestation and avalanche mitigation (Fig. 5). On the upper slopes, structural walls were installed and the slopes were terraced to retain snow. On the lower hillslopes, landowners constructed stonewalls and rock-filled wooden boxes to retain avalanches before they could advance onto the valley ...

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... 6. Geologic cross-section of the Malnant along A – A V transect in Fig. 5.  ...

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... processes can reverse centuries-long channel regimes from aggradation to degradation in mountain watersheds. The Malnant drainage covers 15.9 km within the upper Fier watershed in the Bornes Range within the Haute Savoie of the French Alps (Fig. 4). The drainage is an anticlinal valley underlain by Oligocene limestone and sandstone that have been folded and faulted by alpine tectonics (Figs. 5 and 6). The watershed was glaciated during the Pleistocene and the Little Ice Age. Valley glaciers deepened the fluvial valleys, deposited till, and truncated large alluvial fans (BRGM, 1990). The Malnant is a 7.1-km-long tributary that drops from a headwater elevation of 1835 to 600 m where it joins the Fier River, made famous as a favorite fly- fishing stream of Winston Churchill. The Malnant contributes very little flow to the Fier but does contribute the majority of bed load. Elevations in the Malnant range from 450 m in the valley to over 2000 m on the drainage divide. The watershed receives an average annual precipitation of 1500 mm, including heavy winter snows on the steep valley sidewalls that frequently trigger avalanches. Precip- itation is derived from oceanic rains and by the oro- graphic influence of the Aravis Ridge. The highest monthly precipitation is f 190 mm in December, and the lowest monthly precipitation is 120 mm in April. Snow contributes in water retention with a thickness of 1.80 m at Th ˆnes and 4.80 m in the Aravis Ridge (Cholley, 1925). The Malnant valley was excavated by glaciers during the Quaternary and has been sculpted by slope processes since the Late Pleistocene, including mass movement from the western hogback below Couturier Ridge, avalanches from the hogbacks surrounding the valley, rockfalls from the southern end of the valley, and debris torrents in ravines originating above 1500 m. Heavy flooding and snow avalanches from the mid-14th century to the end of the 19th century created sizeable debris cones at the foot of valley sidewalls. Large woody debris has been recovered from debris cones that yielded carbon dates of 135 F 45 years. Severe floods were recorded in 1854, 1859, 1875, 1879, and 1899, all of which devastated the hamlet of Montremont (Mougin, 1914). This hillslope instability may be attributed to deforestation that provided wood as the primary fuel for residents of Montremont and was utilized by a local glass factory in the village of Allex, active from 1755 to 1860. Deforestation of the original forest of white fir ( Abies alba ) and beech ( Fagus silvatica ) was widespread from the 16th century through the early 18th century. Forests were converted to pastures for summer grazing at elevations above 1500 m, to meadows for summer grazing between 1500 and 1000 m, and to ploughed fields for cereal grains on the valley floor below 1000 m. A detailed land cover map dated 1732, the so-called Sarde Map of Th ˆ nes Parish, was studied by Moutard (1995). This map revealed that forest cover had been reduced to 10% of the present- day cover. Mougin’s descriptions date back to a period when foresters attributed the supposed increased activity of alpine torrents during the 19th century to human action. The French Revolution (1789 – 1799) was supposed to have increased individualism and to have permitted an intense degradation of forests (Surell, 1841). This view was contested in the 1920s by renowned geographers such as Lenoble (1923), who argued that forest degradation had occurred in earlier centuries. This view tended to reduce blame that had been placed on the upland peasantry. More recent works have demonstrated that the Alps have experienced several phases of increased torrential activity during the Holocene and that the Little Ice Age (mid-14th to the end of the 19th centuries) was the most active period. This was probably due to extensive forest degradation performed since the Middle Ages, which was a period of dense human occupation at high altitude in a phase of climate warming until mid-14th century (Ladurie, 1983). Rivers of the Alps experienced a metamorphosis from meandering to braiding in response to an increase of bed load, and this metamorphosis extended downstream from the inner Alps to the piedmont areas surrounding the Alps (Bravard, 1989). However, the 19th century was not homoge- neous since the existence of several low-flow-domi- nated and flood-dominated periods alternated during that century. This periodicity has been demonstrated in the Pre-alps of Die, south of the study area. The years 1800 – 1820, 1840 – 1842, 1850 – 1851, and 1856 – 1875 were particularly severe; and the excep- tional flood of 1856 motivated the French Parliament to enact the 1860’s law on montane reforestation (Bravard, 2000). In response to the hillslope instability and associ- ated problems for valley residents, the French governments purchased the upper 15% of the watershed between the years of 1883 and 1903 and began a program of reforestation and avalanche mitigation (Fig. 5). On the upper slopes, structural walls were installed and the slopes were terraced to retain snow. On the lower hillslopes, landowners constructed stonewalls and rock-filled wooden boxes to retain avalanches before they could advance onto the valley floor. Beginning in the 20th century, sediment supply from hillslopes to the upper Malnant was reduced. Without this sediment input, the Malnant began to incise and bed load was mobilized. In our channel survey of the upper Malnant, the rotor from a British plane that had crashed in the 1950s near the headwaters was found incorporated into the bed load approximately 2 km downstream from the crash site. Downcutting in the last three decades has created severe problems for farmers in this small watershed as bridges are undermined, streamside roads are threatened, and irrigation diversion structures are rendered unusable. Indeed, Malnant trans- lates from French as ‘‘bad creek,’’ ‘‘ . . . a native expression reflecting a secular wisdom’’ (Cholley, 1925). The steep upper Malnant begins at the conver- gence of a series of ravines and avalanche paths that collect water, snow, and sediment from the upper limestone cliffs. The bed of the upper Malnant is characterized by steep, rocky outcrops upstream of 1100 m, by boulder and log steps between 1100 and 950 m, and is incised into till deposits between 950 and 850 m. In the section between 1100 and 950 m, we computed that 18 out of 140 m (13%) of the drop in relief was controlled by log steps. The large woody debris originates from avalanche paths, and this may reflect the increase in supplies of large woody debris following the success of afforestation programs. At an altitude of f 950 m on the left bank of the upper Malnant, we found a piece of wood preserved below a large block belonging to a layer of coarse deposits. This alluvial layer was covered by large fir trees and riverine willows dating back to a period when the torrent was not yet incised. Radiocarbon dating provided the date of 135 F 45 BP. This date is consistent with the period of increased torrential activity described earlier. Floods of the 20th century have triggered incision of the bed of the upper Malnant, and this incision has exhumed large blocks. Recent activity of ravines, still active on the partly protected slopes, has delivered a large amount of small debris (plates of Hauterivian marly limestones) that covered the coarse deposits with a 4-m-thick layer. This debris replenishes bed materials that had been lost by incision. Floods in the Malnant export a portion of this material and move it downstream. This fine debris was excavated in the 1970s by a contractor at the downstream limit of the State Forest (1020 m). This instream mining interrupted the downstream transport and created a knickpoint, which migrated upstream until blocked by large boulders. This excavation stopped 20 years ago, but new deposition since that time has been rather low. The 4-km-long reach between 820 m and the confluence with the Fier River is comprised of gravel, cobbles, and boulders deposited during the high energy floods of the Little Ice Age. The fining of coarse sediment is linked to the reduction in bed slope, but steep tributaries delivered coarse debris until the downstream end of the reach. Downstream of B ́lossier Bridge (km 6.500), the Malnant River has built a large alluvial fan since the Late Glacial. This fan has been incised during the Holocene (undated phase), and a new fan has been emplaced during the Little Ice Age. The material of this late phase of the fan is coarse (gravel and small boulders) if compared to the underlying material, which is composed of small debris. Also, it is partly composed of urgonian boulders originating from the upper cliffs of the watershed, while small debris of the lower layer originate from Hauterivian marly limestones with few urgonian elements. Incision through coarse material was exacerbated when the upper layer was removed and when the fine debris were reached following bed degradation in the 1960s. This reach has been recently degrading because of the upstream reduction in sediment delivery and because of gravel extraction upstream. This process promoted a downstream progradation of erosion. Also gravel extraction downstream along the Fier River promoted regressive erosion to the detriment of the Malnant River bed. Moreover, the tributaries of the downstream reach of the Malnant have been effi- ciently controlled by the French government and are almost no longer active. Degradation was measured on the Malnant from an altitude of 820 m (km 2.500) to the confluence of the Fier River (km 7.100 m, 595 m). Degradation could be dated considering that deflection devices made of concrete were installed at 41 points along the lower 4.5 km of the Malnant in 1968 by the Agricultural Service of the Department of Savoy in order to trap sediment and to control strong lateral erosion (Fig. 7). The spurs were seated ...

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... response to the hillslope instability and associ- ated problems for valley residents, the French govern- ments purchased the upper 15% of the watershed between the years of 1883 and 1903 and began a program of reforestation and avalanche mitigation (Fig. 5). On the upper slopes, structural walls were installed and the slopes were terraced to retain snow. On the lower hillslopes, landowners constructed stonewalls and rock-filled wooden boxes to retain avalanches before they could advance onto the valley ...