The Southern Indian Lake Impoundment and Churchill River Diversion

The 242 000-km2 Churchill River basin extends across the northern half of Alberta, Saskatchewan, and Manitoba. In 1976, hydraulic control structures were completed to divert 75% of the natural river flow of 958 m3∙s−1 across the drainage divide separating the Churchill and Nelson river basins in northern Manitoba. The diversion flows follow 300 km of tributary valleys southward to the Nelson River channel where a 30-yr, 10 000 MW hydroelectric scheme is being developed. The diversion was accomplished by damming the northern outlet of Southern Indian Lake, a 1977-km2 riverine lake on the Churchill channel (latitude 57°N, longitude 99°W). The dam caused a 3-m impoundment above the historical lake levels, which flooded 414 km2 of the backshore zone. Permafrost, or permanently frozen ground, is widespread in the uplands surrounding the lake. As bedrock occurred on only 14% of the postimpoundment shoreline, severe erosion of the frozen backshore deposits is now underway. A long period of instability is anticipated on lake shorelines and in river valleys affected by the altered hydraulic regime. Although the whole-lake water exchange time was increased by only 41% by the impoundment, the circulation patterns and exchange times in individual basins of the lake were changed dramatically when the Churchill waters were diverted at the southern end of the lake. The effects of the changing regimes on the aquatic habitats and fisheries of Southern Indian Lake have been investigated in pre- and post-impoundment studies undertaken by the Freshwater Institute of the Department of Fisheries and Oceans.