The Gebidem Dam located on the Massa River intercepts annually around 400’000 m3 of solid material. These sediments must be evacuated by annual flushing of the reservoir over 4 to 7 days. A trend for silting in the flushing channel was observed during the last decades, leading to damageable overtopping of the lateral walls. In order to improve knowledge on input and output of sediments in the channel of Gebidem, physical and numerical modeling was performed in 1994 and in 2002 at the Laboratory of Hydraulic Constructions of the École Polytechnique Fédérale de Lausanne (EPFL). It was clearly demonstrated that the silting process is related to an anti-dunes regime progressing in the upstream direction from deposit zones which develop initially in the curves and the river confluence with Upper Rhone River. At the opposite, the clearing process starts at the upper limit of the channel and progresses by pushing the sediments downstream. The proposed solution requires an additional water supply to be introduced from the entrance of the channel. In order to optimize the clearing efficiency, the dilution supply can be progressively reduced as soon as the sediment concentration diminishes. Being aware of the importance of the annual flushing, which is the only measure to guarantee sustainable operation of the hydropower scheme, Electra-Massa continuously undertakes efforts to ensure and further improve flushing operations.