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Trent-Severn Waterway National Historic Site of Canada

The Water Management Program

Appendix Three

Severn River Watershed

Lake Simcoe and Lake Couchiching Basin. The Black River watershed feeds into the Severn River downstream of Lake Couchiching. The lakes and channels of the Severn River below Washago include Sparrow Lake, Six Mile Lake, and Gloucester Pool.

Size: 6,160 square kilometre drainage area
Physiography: Lake Simcoe and Couchiching Basin are mostly rolling farmland with deeper soils. The Black River watershed is located in the thin soils and rock of the Precambrian shield.
Water Sources Rain, snowmelt, and ground water

Runoff Evaporation
Lake Simcoe – Couchiching Basin: Runoff is slow for the Lake Simcoe and Lake Couchiching Basin. Only 25% of the precipitation falling on this watershed eventually appears as a run-off flow. The basin drains into Georgian Bay. It receives high snow fall. Evaporation losses from both land and lake surfaces are high. Long-term average flow is 26.7 m3/s for the Lake Simcoe and Couchiching Basin.
Black River Watershed: Joins the Severn River below the outlets of Lakes Simcoe - Couchiching at Washago. The Black River watershed is virtually unregulated and produces rapid run-off of precipitation with lower evaporation losses. Long-term average flow is 21.7 m3/s with high peak flows during the spring period. The maximum recorded daily flow at the Black River gauge station for the period 1915 to 2006 was in excess of 225 m3/s. The lowest summer minimum daily flow was only 0.4 m3/s.

Dams: Controlled by several small dams in the Washago area. A lack of capacity makes operating Lake Simcoe difficult.

Lakes Simcoe and Couchiching are managed using a rule curve that has been in effect since 1918. This curve serves as a target or guide for water levels throughout the year.

Lake Simcoe Level Summary

Winter: Outflow from Simcoe-Couchiching is regulated to maintain the lake near its rule curve, which is a constant level from November 20 to March 20.
Spring: The two main spring objectives are to fill Lakes Simcoe-Couchiching without overfilling, and to minimize the effect of the peak flow from the uncontrolled Black River. Simultaneous peaks on Lakes Simcoe-Couchiching and the Black River can result in flooding on both Lakes Simcoe-Couchiching and Sparrow Lake. The outflow from Lake Couchiching is restricted or shut off as the Black River peak passes, in order to mitigate flooding in Washago and on Sparrow Lake.
Summer: During the summer, levels are managed to the rule curve.
Fall: Typically, Simcoe-Couchiching drops to its lowest level, but is maintained above historic minimums.

Lakes Simcoe-Couchiching form a very large body of water and levels take a long time to respond to adjustments. If the lake levels are higher than normal, higher outflows will be maintained. If the lake is lower than normal, then lower outflows will be maintained. Flows from Simcoe-Couchiching must also be coordinated with flows coming from the Black River to reduce the threat of high water on the Severn River and Sparrow Lake. The natural watercourses of the Black and the Severn Rivers are constrained by numerous narrow sections, which cause rises in river levels during high flows.

Flooding: The Black and Severn Rivers are prone to rapid and high runoff of spring flows from an area with very little controllable storage. The sudden release of water often overtops the Lake St. John dam, aggravating local conditions and flooding residents along the Black River down to Washago. Sparrow Lake flooding is partially caused by constriction in the Severn River at Sparrow Lake Chute, McDonald's Rapids, and Hydro Glen. The cost of removing these restrictions, however, would be prohibitive compared to actual economic losses due to flooding, which would not be wholly eliminated by the excavations.

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