Dams change the way rivers function. They can trap sediment, burying rock riverbeds where fish spawn. Gravel, logs, and other important food and habitat features can also become trapped behind dams. This negatively affects the creation and maintenance of more complex habitat (e.g., riffles, pools) downstream.
the dams will bring more problems than they will solve. Hydropower dams flood large areas, force people to relocate, threaten freshwater biodiversity, disrupt subsistence fisheries, and leave rivers dry – substantially affecting the ecosystem.
To keep a dam economical we have to calculate the ecological flow for high and low flow season. In this way we can keep the downstream ecology functioning.
these are the River continuum and the River discontinuity concepts explaining dams' influence on the river. The reference of the question is huge. I prepared two large articles on the influence of a dam in the small mountainous river on communities of free-living ciliates – species content, structure, and functional activity... (in press in Poland and Bulgaria). For more detailed information look Attachments.