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

Abstract

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 km 2) located in the Eygues River basin (1150 km 2), a tributary of the Rhône 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.