How Dams Affect Water and Habitat on the West Coast
Changing a habitat from a river to a lake can have many negative effects on fish.
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 is especially true for activities such as spawning.
Predator/Prey Relationships and 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 a dam are often the targets of predators. Changes to habitat may benefit predator species allowing them to increase their numbers. For example, the northern pikeminnow, a native predator, prefers slow water habitat.
In the Columbia River’s natural state, this type of habitat was relatively rare. 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 get trapped in the reservoir behind the dam, whereas 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 that use these substrates and pools.
Water Quality and Quantity
The purpose of many dams in the Pacific Northwest is to collect and store water for uses such as hydropower and irrigation. Water diverted from the river results in lower natural flows and less habitat for fish downstream. In addition, changes occur in the quality of water when it is stilled behind a dam.
Flow timing
Fish are adapted to a particular natural pattern of flows in a river. In the northwest, most rivers have peak flows in the spring and lower flows in summer. The way dams use water often results in changes to these natural patterns. Reductions in peak flows may inhibit the formation of pools and riffles and other habitat types that are important to fish
Ramping
There may also be problems if dams suddenly release water or reduce flows causing river levels below the dam to rise or fall suddenly, potentially stranding fish. This is sometimes done to answer the needs of power generation, water is stored in the reservoir during periods of low power demand and then released later to generate electricity when demand is high.
Temperature
Water held in reservoirs tends to heat up, increasing the temperature of the river. Salmon and steelhead prefer cool water. However, water on the bottom of the reservoir remains cold through the summer. If cool water is taken from the bottom (some dams cannot do this) it may actually be used to help keep the river cool through the summer.
Total dissolved gas
Falling water, such as from a dam spillway, may mix nitrogen from the atmosphere into the water. Water can only hold a certain amount of nitrogen, when this is exceeded, bubbles form. These bubbles can also form inside the bodies of fish which are swimming in the water causing injury or death.
Oxygen
Water released from the bottom of the reservoir is often low in oxygen, causing problems for fish downstream. However, water falling over a spillway may actually mix more oxygen into the water.
Toxic substances
Sediment settles out in reservoirs behind dams. Many toxic substances may be trapped in these sediments such as pesticides, or heavy metals from mine tailings. If these sediments are disturbed these substances may be released into the water.
Eutrophication
Reservoirs often produce large amounts of algae and other plants. The result is higher levels of nutrients in the river downstream. Too many nutrients can cause problems such as low oxygen levels or excessive growth of algae.