Dammed rivers release less water because of increased evaporation. Another problem is riverbed deepening which lowers the groundwater table. Where the water pools up is also important - having a bunch behind a dam when former wetlands and floodplains are dry is obviously not conducive to the ecosystems in those wetlands and floodplains.

Doesn't that all very heavily depend on the exact nature of the system the dam is part of? We all know of the environmental problems of the Assuan dam in Egypt. But that does not mean that properly planned dam cannot be very beneficial. One shouldn't just build a dam where it is convenient but with careful analysis first. There are plenty of places where a dam just means less cities flooded occasionally but otherwise has little eco system impact.

Why do dams increase evaporation? Having the water all in one place as opposed to distributed over the length of a river should reduce surface area.