Ultimately, dams do not stopthe water, but they do stopvirtually all the sediment from moving downstream. This causes the reservoir to fill up with sediment reducing the volume of the reservoir. This also starves the downstream sections of the river and coastline of the sediment they depend on for maintenance of channels, islands, deltas, and other landscape features.

Dams prevent fish from swimming upstream to their spawning grounds. If fish cannot migrate and spawn, then their population decreases. People have attempted to remedy this problem with different approaches, but most commonly implemented is a fish ladder which creates a stepped system. The fish jump onto each step to safely surpass the dam.

Dams create quiet lakes where delicate silica-loving diatoms (micro-organisms which are pictured above) can grow. This takes silica out of the river. When water reaches the ocean, there is little silica left for diatoms in the ocean. Without silica, the diatoms do not bloom and harmful algal blooms take their place.