Effects of Impoundments

We have many dams on Ontario streams (1,596 medium and large dams*). Too often, discussions of watershed management suggest only control structures such as dams, but few fundamentals such as watershed habitats, particularly headwaters, and stream conditions. Dams don’t just control floods; dams have other impacts both upstream and downstream.


Impounding water upstream of dams and weirs increases the area of surface water exposed to the sun and wind. It also increases the area of shallow water in intimate chemical and physical contact with the flooded earth and the organic litter. The increased contact between shallow water and the organic soil and litter increases the amount of mercury that is released into the water. The amount of solar energy absorbed by the water depends on the surface area exposed to the sun and the duration of exposure. More solar energy absorbed means higher temperatures of the surface water and greater depth of the heating.


Water pooled above dams will be heated to 0.5 m deep or more over summer whereas an undammed creek’s surface water flows so it is exposed to the sun for much less time and is heated much less. Impounded water may reach 70 to 80 degrees F (21 to 26 C) when the creek could stay cooler than 70 degrees F (21 C). The heated impounded water would no longer support fish that need cool habitat such as speckled trout. Fish such as smallmouth bass would take over. If the heating of the impounded water is less severe, non-native brown trout or rainbow trout may be introduced from hatcheries. These fish often have been selected in hatcheries to tolerate higher temperatures than native speckled trout could survive. Walleye (pickerel) also have been stocked into habitat with temperatures warmer than trout habitat but cooler than smallmouth habitat provided there are suitable spawning grounds. Impoundments and our responses to them change the fish fauna.


The notion that dammed pools will provide a refuge for fishes when drought dries up the flowing stream below the dam is false hope. If cold-habitat fish were still in the creek, they would not survive in the warmed waters of the impoundment. And if downstream fish should attempt to take refuge in the impoundment they would need to jump over the dam or weir to get into the pond. Impoundments are refuges only for impoundment fish.


The increase in solar energy absorbed by impounded water also has major effects downstream from the dam or weir. Because most dams or other control structures allow downstream flow only over the top of the dam, the water that flows downstream is the hottest water from the surface of the impoundment. This flow of warmed water wipes out all cool-water habitats for some distance downstream. Unless the stream is well shaded, most of the stream may be affected. This effect also lowers the rate that oxygen will dissolve into and be held by the stream water. And the effect is not just on fishes. Cold water Invertebrates and algae also are eliminated.


The increased heating of impounded water also increases the rate of evaporation, increasing the total loss of water from the watershed. This is not helpful in adapting to the droughts of climate change. Evaporation depends mainly on the amount of heat stored in the impounded water and the area of water surface exposed to the air. Damming a narrow stream, turning it into a large area of warmed water moves more surface water back into the atmosphere.


Dams and other control structures are barriers to the movement of many species (e.g. Castric et al.). This affects migratory movements and movements to spawning grounds. Upstream movement of Lake Ontario’s Atlantic salmon and upstream and downstream movement of American eels in Canadian rivers exemplify the spawning problems.


Perhaps a more critical impact of dams is fragmentation of populations and gene pools. Structural fragmentation of brook trout streams was shown to cause local extinctions of the fish in less than six generations**. Loss of small, fragmented populations and of genetic types will make difficult the adaptations to variations in stream conditions that will be demanded by climate change.


Our watershed management should no longer rely on dams and other control structures. We should consider removal of dams that no longer have an economic or social rationale. We should rely more often on stewardship of entire watersheds. There are many socio-economic and ecological benefits to managing watersheds by applying good land and water stewardship to the watershed ­– especially the two-thirds of the watershed that is headwaters.





Castric, V., F. Bonney and L. Bernatchez. 2001. Landscape structure and hierarchical genetic diversity in the brook charr, Salvelinus fontinalis. Evolution 55: 1016-1028.

* www.State of Ontario’s Biod#308523B

** www.sse.gov.on.ca/….webloc

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