Geomorphic changes of a dry-temperate meandering river highly disturbed by a large dam, Sauce Grande River, Argentina

International audienceFlow regulation by dams may modify the morphology of the river downstream by altering the equilibrium between the flow transport capacity and the sediment load available to transport. This study focuses on the Sauce Grande River, an ungauged dry temperate basin of about 4000 km² located in central-eastern Argentina. The Paso de las Piedras Dam has impounded the middle river section since 1978 for water supply to Bahía Blanca (400 000 inhab.); despite the large capacity of the impoundment, the effects of the dam on the river environment remain poorly evaluated. This study quantifies the geomorphic response of the river downstream to major decrease in discharge and sediment inputs, and compares the direction of geomorphic changes to qualitative models of channel adjustment. River channel changes were inspected based on (i) field surveys of the present river channel state, and (ii) historical aerial photographs and imagery of pre-dam (1961) and post-dam (1981; 2004) channel states for two river segments of 40 km-length above and below the dam. Methods used GIS-based measures of channel narrowing (widening), lateral migration and vegetation growth between consecutive time steps, and field-based morphologic descriptions of sample reaches along the river segments. Results showed that geomorphic changes within the river below the dam were related to human intervention (e.g. bed digging, artificial meander cutoff and levees building) more than to natural adjustments to reduced flow discharge and sediment load (e.g. incision and narrowing). In contrast to the river upstream, which exhibited high lateral mobility, there was no clear evidence of channel activity since dam closure; bank profiles showed high stability and progressive vegetation growth. These findings suggest that the dam has induced stable (moribund) river channel conditions downstream. Results are discussed relative to predictive models of channel metamorphosis below dams available in literature