Well, they definitely change the environment, and changes often kill off something while promoting the life of other things. In the case of dams, they raise the level of the water behind the dam, decrease the amount of variation in water level in the rivers and streams below the dam. The also hamper fish migration both up and downstream. I think if you googled for hydroelectric dam environmental damage you'd probably get a lot of good hits.

It completely throws off the balance of nature and many species of plants and animals die,or have to find new areas to move.Farmers loose the productivity of their farmlands.These dams are very much unlike the beaver dams that help that help the environment not destroy it for short term selfish goals.

I'm doing my graduation project on this so I know a lot- the Colorado River is my focus, so I'll just use that. -It changes the temperature of the river- the Colorado River used to be around 80 degrees F year round, now it's only 42, which is killing native fish. -The river does not carry as much sediment. It used to carry around 500,000 tons of sediment each day. Now all that sediment is sitting on the bottom of Lake Powell, a unnatural lake created by a dam. -It kills native fish- These changes kill native fish, so man has added non-native fish such as the Rainbow Trout, which just kill more native fish. Some of the native fish are already endangered or extinct. -It causes beaches to go away. The lack of yearly floods have caused to beaches to turn into more grassy lands. One year, they actually had an artificial flood because they were afraid that this would happen. I could go on, but I think you got the idea. For more here are the sources that I used. http://www.classzone.com/books/earth_science/terc/content/investigations/esu401/esu401page01.cfm http://esciencenews.com/articles/2008/07/10/river.damming.leads.dramatic.decline.native.fish.numbers http://www.bobspixels.com/kaibab.org/misc/gc_coriv.htm and these books Damming Grand Canyon by Diane E. Boyer and Robert H. Webb The New River Controversy by Thomas J. Schoenbaum