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There are several dams and diversions upstream of the South Saskatchewan River Basin (SSRB), particularly in the Bow River and Oldman River sub-basins.

Dams and weirs have several important effects. First, they change the portion of the river channel where the project is constructed from a river environment to a lake environment. Reservoirs inevitably change distribution and abundance of aquatic biota. Nutrients leached from flooded soils may actually increase biological productivity, albeit for species different from the original dominant inhabitants. Even when a project provides little storage, such as a run-of-the-river hydro-facility, dams and weirs fragment the natural ecosystem by providing barriers to migration of aquatic species and disrupting riparian habitat.

Reservoirs store water during the spring runoff and release it for later use. Hydroelectric stations tend to be used to meet peak load requirements, while thermal generation is used to meet base loads. Although peak power demands occur on a daily basis, hydroelectric stations tend to reverse the high and low cycles of the natural hydrograph. As overall electricity demand is greater in the winter than in the summer, river flows can be higher than natural in the winter and lower in the summer.

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stmaryWhile hydroelectric projects consume no water other than losses to evaporation, they do significantly alter the flow pattern. Because irrigation water demands are in the summer, flow conditions downstream may in some cases mimic those of the natural hydrograph when water is released from storage for use downstream. Irrigation water demands, however, are significant enough to reduce downstream flows, thus raising concerns about instream flow requirements for aquatic life.

River channels immediately downstream of a dam will tend to scour because the water discharged from the reservoir carries less sediment than the pre- project water. Further downstream, the absence of a spring peak flow (except under very high runoff conditions) reduces the natural flushing of sediments deposited in the river channel, thus changing the character of the river channel. In particular, permanent vegetation is likely to develop in areas now subject to decreased annual flooding. Conversely, riparian vegetation, such as cottonwood trees, requires periodic flooding to sustain new growth. Decreased flooding inevitably leads to loss of the riparian forest. Some aquatic species are sustained by periodic flushing of the channel.

Reservoirs also affect the thermal regime of the river. Summer releases tend to be cooler than normal, while winter releases are warmer. Water temperatures will affect the distribution of fish species downstream of a reservoir. Changes in the winter flow regime will change ice conditions in the stream and affect the winter aquatic ecology.

The physical, biogeochemical, and biological processes within the reservoir will affect water quality in the reservoir and downstream. The degree to which water quality is affected will depend on factors such as the surface-to-volume ratio and depth of the reservoir, surficial geology and soils of the catchment, sedimentation rates, magnitude and timing of flows entering the reservoir, and biological productivity of the reservoir.

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Dams and Diversions in the South Saskatchewan River Basin

Dams are developed for several uses such as flood and drought control; and power and water consumption. For instance, the Oldman River dam is for drought control. Since dams control the quantity of water, they can have negative impacts to riparian areas. There are six dams on the Bow River, three on the Oldman River, and one on the Red Deer River. All research areas are impacted by the upstream dams.

For 75% of the SS-01 area, from where the Bow and Oldman Rivers join to form South Saskatchewan River to the Medicine Hat gauging station, 25-50% of the watershed is controlled and impacted by upstream dams. In the SS-02 area, from the Medicine Hat Gauging Station to the Alberta / Saskatchewan border, over 50% of the watershed is controlled and impacted by upstream dams.