How do hydroelectric dams affect the ecosystem?

Hydroelectric dams affect the ecosystem in many ways, dependent in part on the ecosystem itself and on the dam itself. Generally, the effects are negative.

Damming a water source drastically alters the flow of that water source. It can change the direction of the river, the depth of the river, the temperature of the river, the width of a river, and so forth. These changes have consequences for the organisms inhabiting the adjacent environments.

The image below shows a before and after of a large dam in between Paraguay and Argentina.
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As you can see, the rivers have been replaced by a large lake. Animals that required certain river conditions will have moved on to find other suitable habitat and they may or may not have been successful.

The plants in the river will also drastically be affected and thus any organisms that depend on those plants for food or shelter will need to adapt or they will die out.

The migration route of fish and other aquatic animals will also change, which in turn could affect the animals that consume them.

The water in rivers typically run faster than the waters in lakes. Thus, the lake water will accumulate more sediments and nutrients, changing the composition of the water and potentially making it unsuitable to any organisms that need lower sediment content.

You can read more about the effects of hydroelectric dams here.