Morphodynamic changes in a natural river confluence due to a hydropower modified flow regime

River channel confluences form important morphological components of any river system. This article presents the results of an investigation on a natural river confluence in Switzerland. The aim of the field work performed at the confluence of the Sarine (main channel) and Gerine (tributary) Rivers near Fribourg (CH) is to analyze the interaction between morphological processes on the confluence and hydropower-affected hydrology of the main River. Important morphological changes have been observed over the last years, especially the deflection of the Sarine River and erosion of the left bank opposite of the tributary. Numerical calculations have shown that currently the minimal discharge of the Sarine River which transports the sediments annually deposited at the confluence is attained on average once each 2 years. Before the construction of the Rossens Dam, the minimum discharge was reached every year. This lack of yearly floods with sediment mobilization can explain the morphological changes occurred at the confluence since the construction of the dam.