Water Quality & Quantity

The purpose of many dams in the 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


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.

River flows With and Without Dams 59kb

Ramping 60kb


Water in 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.


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.


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.