The Shellmouth Reservoir (also known as Lake of the Prairies) is a man-made reservoir on the Assiniboine River in Manitoba and Saskatchewan, Canada. The Shellmouth Dam (50.96361°N, -101.41861°W) is a multi-purpose embankment dam built by the Prairie Farm Rehabilitation Administration (PFRA). The dam and reservoir are part of a strategy to reduce the risk of flood damage for Winnipeg and other communities along the Assiniboine River. For example, in the 1997 Red River flood, the inflow to the reservoir peaked at 10000 cuft/s while the outflow never exceeded 1700 cuft/s. The reservoir is also used to supplement flows on the Assiniboine when conditions are dry, ensuring water supply for Brandon, Portage la Prairie, irrigators, and some industries. For example in early August, 2021 the flow in Brandon was a little over 9 m3/s while the outflow from Shellmouth was 5.7 m3/s and inflow was less than 0.5 m3/s. Therefore without the dam the flow in the Assiniboine at Brandon would be a bit under 4 m3/s. Asessippi Provincial Park is established around the southern arm of the lake.

Specifications

The dam is 70 ft high and 4200 ft long. The reservoir is 35 mi in length and stores 390,000 acre feet (480 million cubic metres) at the spillway crest level of 1408.5 ft. Outflows are controlled by a gated conduit and a 210 ft wide concrete chute spillway. The normal summer level target is between 1400 ft and 1404 ft.

Fish species

Fish species include walleye, yellow perch, northern pike, mooneye, burbot, rock bass, brown bullhead, white sucker, shorthead redhorse and common carp. Rock bass are Saskatchewan's only native bass.

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