For the first time in France a dam is being removed in order to restore ecological and sediment continuity and to allow salmon migration. This modification of the dam of Poutès configuration allows the establishment of a long-term study. The first step is the evaluation of the geomorphological, sedimentary and ecological characteristics before the works in a reach of 21km including the bypassed section. This analysis is based on the study of diatoms and benthic macroinvertebrates collected upstream of the dam, in the bypassed section, and downstream of the penstock pipe release. The results show the impact of the dam on communities arising from a change in the water quality, but also habitats including the presence of very long and deep pools in the bypassed section, which alternate with rapids, which change the river dynamics of watercourse.