Impacts of Dam Construction and Operation on Fish & Wildlife


Loss of River and Stream Habitat

The flooding of lower-elevation creeks and rivers decreases the amount of spawning, rearing, and over-wintering habitat for a variety of fish species including kokanee and rainbow trout.

Barriers to fish movement

Dams provide a physical barrier to upstream migration. Anadromous fish who spend much of their life in the sea water but come to freshwater to spawn( such as salmonids) are affected In 1941, the completion of the Grand Coulee Dam on the Columbia River in Washington State resulted in a permanent barrier for three species of ocean salmon that once swam north into Canada to spawn.. There are many other species affected including trout, burbot, and sturgeon.

Changes in Productivity

Dams result in an initial increase in the level of nutrients in a newly created reservoir, due to the flooding, and subsequent decomposition of plant matter that releases nutrients into the water. This will help a variety of fish species temporarily.

Once this initial burst of productivity is over, the reservoir can become oligotrophic (waters that are relatively low in nutrients). This can be for a couple of reasons: upstream dams acting as a barrier to nutrients flowing downstream (see impounding nutrients) and/or a change in water velocity. Free-running rivers carry sediments throughout ecosystems. In dammed rivers, the sediments fall to the bottom of the reservoir and are removed from the food web. .

All reservoirs decline in fish productivity over time. Nutrient retention occurs for the first 20 to 30 years after impoundment, and then it declines. The less nutrients there are in a body of water, the less species diversity.

Changes in Water Clarity

Reservoirs usually have greater water clarity than rivers as the lower water flow allows sediment and nutrients to sink to the bottom. As water clarity increases there can be an increase in light penetration (compared to a fast flowing river) promoting phytoplankton (microscopic plant matter) growth.

The changing water clarity can have a direct impact on a number of fish species. For piscavores (fish-eating fish) rainbow trout, gerrards, bull trout etc. there is a greater opportunity for them to see their prey. The reverse is true for species that are trying to elude the predator fish, such as kokanee.

Impounding of Nutrients

Dams create a physical barrier to nutrients flowing downstream. Natural phosphorous and nitrogen (eroded from the mountains) are trapped upstream of a dam, sinking to the bottom of a reservoir and being removed from the food web.

Changes in Water Temperature

The water downstream of the reservoir is affected by upstream storage. If the water released is from the reservoir's deeper, cooler levels, the water temperature downstream can be lowered; if the water is taken from close to the sun-warmed surface, the downstream water temperature may be increased, especially in summer.

There are also impacts caused by dam operations such as dissolved gases. Gases are dissolved when water comes into contact with the air as it goes over the spillway, trapping nitrogen and oxygen from the air in the deep spilling basin below. The gases can cause a potentially fatal bubble disease in the blood vessels of fish, like a diver experiencing 'the bends'


Land and water have a close ecological relationship. The effects of dam construction on wildlife may not be as obvious as those on fish but they exist. These can include:

Loss of Habitat

Loss of low elevation, valley bottom habitat which is often very productive and rich in species diversity. Sometimes reservoirs help create more wetlands but more often the valley floor that is flooded results in a net reduction in wetlands, riparian areas and open forests.

Many species, including deer, moose and elk, depend on lower elevation habitats for breeding and over-wintering – these in turn support the predator species such as cougars. When these areas are flooded, ecological losses can be registered.

This loss of habitat can affect a wide variety of creatures, and not just those that are usually associated with low elevation, wetland habitat, such as amphibians. Examples include bats that come down from higher elevation roosts to feed on insects in wetland areas; even mountain goats come down to the valley bottoms to seek out natural mineral licks.

Degraded Habitat/Reduced Production

Dams reduce or eliminate normal flooding cycles, effectively isolating the river from its surrounding floodplain. During freshet (spring/summer run-off) the dams result in an absence of nutrient deposition on the floodplain and riparian areas, therefore reducing fish, insect and other wildlife productivity in these areas.

Changes in Productivity

Many terrestrial creatures depend on aquatic productivity. Local examples include ospreys, bears, bald eagles that benefit would be kokanee stocks which went though a decline after the dams were constructed.

Barriers to movement & migration

Large reservoirs can create a major barrier to migrations, both large and small, leading to population fragmentation and a loss of species diversity. The large bodies of water can disrupt traditional movement patterns for migratory species such as caribou; loss on rivers due to dam operation through the winter can also increase mortalities.

Changes in Micro Climate

Bodies of water with a large surface area may make the local climate damper with warmer winters and cooler summers. This can effect the vegetation and species diversity around the reservoirs.

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