Shellmouth Reservoir

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Lake of the Prairies
Shellmouth Dam
Location	Manitoba
Coordinates	50°58′35″N, 101°24′26″WCoordinates: 50°58′35″N, 101°24′26″W
Lake type	reservoir
Primary inflows	Assiniboine River, Shell River, Cupar Creek
Primary outflows	Assiniboine River
Basin countries	Canada
Water volume	480,000,000 m³
Surface elevation	425 m (1,394 ft)

The Shellmouth Reservoir is a man-made reservoir in Manitoba, Canada. It is located near the border with Saskatchewan

The Shellmouth Dam was built by PFRA. Construction started in 1964 and was completed in 1972 costing $10.8 million. The dam is 70 feet (21 m) high and 4,200 feet (1,300 m) long. The reservoir is 35 miles (56 km) in length and stores 390,000 acre-foot (480,000,000 m³) at it full supply level of 1,408.5 feet (429.3 m). Outflows are controlled by three gated conduits and a 210-foot (64 m) wide concrete chute spillway.

The dam and reservoirs are part of a strategy to reduce the risk of flooding on the Assiniboine River and in Winnipeg. For example in 1997 the inflow to the reservoir peaked at 10,000 cubic feet (280 m³) per second but the outflow didn't exceeded 1700 cfs. The reservoir is also used supplement flows on the Assiniboine when conditions are dry ensuring water supply for Brandon, Portage la Prairie, irrigations and some industries.

The reservoir is also known by the name "Lake of the Prairies". Asessippi Provincial Park is established around the southern arm of the lake.