Norbert Landon

Channel response to increased and decreased bedload supply from land use change: contrasts between two catchments

Abstract The catchments of Pine Creek, Idaho, USA (200 km 2 ), and the Drome River in the Drome Department, France (1640 km 2 ), illustrate contrasting changes in land use, bedload sediment production, and channel response. Hard-rock mining began in the catchment of Pine Creek near the end of the 19th century and, together with road construction, timber harvest, and historically heavy grazing of uplands, resulted in increased tributary bedload yield. Increased bedload migrating to the channel, combined with removal of large cedar trees on the floodplain, resulted in channel instability, which propagated downstream over a period of decades. On many reaches of Pine Creek, active channel width has increased by over 50% since 1933. Over roughly the same time period, the Drome River catchment was extensively reforested (after at least one century of denudation and heavy grazing) and numerous check dams were constructed on torrents to reduce erosion. As a result, the Drome River has experienced a reduction in bedload sediment supply since the late 19th century. In addition, gravel has been extracted from some reaches. Consequently, the channel has degraded and gravel bars have been colonized with woody riparian vegetation. Channel widths in wide, braided reaches decreased from 1947 to 1970 by 60%. On Pine Creek, channel instability has resulted in bank erosion (exposing contaminated mine tailings) and increased flood hazard. On the Drome River, degradation has undermined bridges and embankments, and lowered the water table in areas dependent on groundwater for irrigation, resulting in loss of 6 million m 3 of groundwater storage since 1960. Though they differ in drainage area by nearly an order of magnitude, Pine Creek and the Drome River provide an excellent contrast in that they represent two sides of an epicycle of alluvial sedimentation set off in each case by land disturbance. In both cases, the most recent channel changes, though in opposite directions, were viewed as negative by river managers. On Pine Creek, managers have removed (or protected from erosion) mine tailings, and have attempted to train the stream into a more stable channel, and most rock waste piles (the principal sediment sources) have recently been controlled. On the Drome River, managers have prohibited gravel mining and adopted new policies to permit coarse sediment to migrate through the river system.

Contemporary changes in sediment yield in an alpine mountain basin due to afforestation (the upper Drome in France)

Abstract The Baurieres plain within the upper Drome River basin was used to reconstruct recent changes in sediment supply in relation to changes in land use within an alpine catchment. A considerable body of archival information is available. Furthermore, the plain acts as a natural sediment trap and the reach–basin interaction has not been disrupted by human activity. Based on archival data, channel geometry measurements, dendrogeomorphological and radionuclide analysis (Cs137 and unsupported Pb210), the trends in channel change and sediment supply over the past two centuries are assessed and their causes are interpreted. Dendrogeomorphology and radionuclide profiles show that the floodplain is characterised by a decrease in sedimentation rate in the 1960s. The ex-Pb210 profiles also suggest a spatial modification of the relative contribution in sediment supply of the catchment. Bedload yields are estimated to be at least 26 m3 km−2 year−1 between 1928 and 1996 based on an estimate of storage over a channel length of 11.5 km. Archival data concerning bedload removal from traps yields an estimate of 25.6 m3 km−2 year−1 between 1993 and 2001. These two values are very comparable suggesting no major modification in bedload transport within the reach during the 20th century. If the bedload supply has not strongly decreased in the studied reach during this period, the bedload sources have changed. The volume of sediment stored in the Beaurieres area between 1928 and 1996 corresponds to at least 40% of the sediment delivered by channel degradation from an upstream alluvial reach. Both changes in floodplain sedimentation as well as changes in bedload and suspended contributions from catchment sources are interpreted as responses to land use but also flow regime. Hillslope sediment production strongly decreased due to planned hillslope afforestation and torrent regulation at the end of the 19th century, and spontaneous hillslope afforestation resulting from grazing decline, mainly in the two decades following World War II. The observed change in suspended sediment supply which occurred around 1960–1965 has a clear synchronicity with spontaneous catchment afforestation following World War II. After this period, a decrease in sediment supply, a change in source, but also a decrease in peak flow, were observed. Changes in run-off are complex and cannot be caused with only land use change. Flood hydrographs underwent peak decreases and duration increases through the 20th century because of increase in water retention capacity of the forested catchment. In addition flood seasonality has changed, with September and October flood events being much less frequent in the last part of the 20th century.

Contemporary channel changes in the Eygues basin, southern French Prealps: the relationship of subbasin variability to watershed characteristics

The southern French Prealps are sub-Mediterranean mountains in which forest cover has increased since the end of the 19th century. Forest development, linked with agricultural decline and planned reafforestation, has induced a decrease of bedload supply from hillslopes. In response to this evolution, several channel changes are observed in mountain streams: channel narrowing, channel degradation and pavement development. The intensity of these adjustments differs from one stream to another. Twenty catchments (10–100 km2) located in the Eygues River basin (1150 km2), a tributary of the Rhone River, are studied to highlight the subbasin variability of channel changes and to give causal interpretations. We conducted field surveys and analyzed aerial photographs to characterize present channel morphology and contemporary channel changes and GIS was used to quantify morphometry, relief, land use and geology of the watersheds. Multivariate statistical analysis determined relations between channel changes and watershed characteristics. The potential bedload supply of these mountain streams is controlled largely by watershed morphometry and land use. The most active tributaries of the Eygues River are characterized mainly by high drainage density and poor vegetation cover linked to agricultural land use.

Assessment of Bedload Delivery from Tributaries: The Drôme River Case, France

This paper examines the potential to reverse bed degradation trends in the Dr6me River (southeast France), a prealpine gravel-bed stream, by assessing coarse sediment supply in tributary subbasins. Two approaches are developed. First, a Principal Component Analysis was performed on 24 subwatersheds to characterize the most productive ones. Second, fieldwork and aerial photographs provide detailed assessment for two mountain streams, one characterized by abundant bedload (the Esconavette Creek), the other characterized by 30 yr of inactivity (the Barnavette Creek). Active tributaries (with developed active gravel bars) have been discriminated according to variables describing watershed sediment storage and production potential. Variables describing human intervention on erosion and vegetation cover appear to exert secondary influences on geomorphic activity in a context of general geomorphic stabilization. Declining activity associates features such as bed incision, channel narrowing, and vegetation development in riparian buffer strips in the two mountain streams studied. They are more frequent on the Barnavette Creek, the inactivity of which is partly explained by smaller lateral inputs from valley-filling.