The Cowlitz and Lewis system are good examples of habitat destruction via damming.
Many moons ago there was an article in STS that compared a similar sized, natural system in AK to the Cowlitz.
Healthy returns aside, one of the glaring differences was the native trout populations.
The AK system had normal populations while the barren (of salmon spawn and carcass) Cowlitz had scrawny, under populated reaches.