Ecosystem Modification

Changing a habitat from a river to a lake can have many negative effects on fish. The presence of the dam may also change the way predators and prey interact. In many cases the negative effects of these changes are greater than the direct effects of the dam itself.

Loss of Habitat

Most salmon are adapted to living in rivers so changing their habitat to a lake often has negative consequences on their life cycle. This especially true for activities such as spawning.

Predator/Prey Relationships & Non-native Species

Changes to the river caused by dams and reservoirs may actually benefit predators while making salmon more vulnerable. Fish delayed while trying to pass dam are often the targets of predators. Changes to habitat may benefit predator species allowing them to increase their numbers. The Northern Pikeminnow, a native predator, prefers slow water habitat. In its natural state, this type of habitat was relatively rare in the Columbia River. Now there are many reservoirs and much more slow water habitat for the Northern Pikeminnow so there are many more pikeminnow to eat juvenile salmon. Changes to habitat may also allow non-native species to invade. These species frequently compete with or prey upon native species.

How Rivers Create Habitat

Rivers are very dynamic systems, always changing shape and moving things from the headwaters downstream like giant conveyor belts. Dams block these processes. Substrate (including sand, gravel and rocks) and large pieces of wood are trapped in the reservoir behind the dam; while downstream of the dam they continue to be carried away. The river below the dam may lose spawning gravel and without large pieces of wood to help form pools the stream channel becomes straight and ditch-like. This means there is less habitat available for juvenile and adult salmon.