The concrete is a small part of the problem. Flooding gigantic areas and stopping the natural water flow have serious consequences for widelife, but most people don't care enough.
Actually, I addressed that in my comment. There are no real serious consequences for wildlife, instead, the local ecosystem changes and different wildlife moves in.
You do realise that rivers are constantly shifting, disappearing, lakes forming and disappearing, that none of this is static, yes?
I do. There is quite the delta between a immense & sudden reservoir and a meandering river shifting its (otherwise similarly sized, and unintterrupted) bed over decades.
I'm not even touching on the human concerns regarding population displacement.
You do realise that rivers are constantly shifting, disappearing, lakes forming and disappearing, that none of this is static, yes?