Every single river contains sediment as an important natural part of its flow. This sediment tends to settle down on the river ground. Erosion by flow hydraulics is working the other way. Usually over the year sedimentation and erosion level each other out. The river keeps its shape.
Installing a reservoir profoundly changes this balance. The reservoir now keeps most of the sediments. Only a part of the sediments is passed on. The downstream river is affected by erosion. This is also valid for pump storage plants in a similar way.