What is it? Drawdown is when lake levels are lowered in the Trent-Severn watershed. It's a process that takes place annually (typically in October) in preparation for winter. It's meant to take into account the timing of fish spawning, levels required for flood mitigation, projected fall and winter precipitation levels, downstream topography and volumes and flow rates. (Source: Parks Canada)
At left, extremely low water level at Burleigh Falls, May, 2020.
DRAWDOWN IMPACTS TO FISH AND WILDLIFE
(Including species at risk)
Winter drawdown to average levels in a “normal or average winter” exposes the most productive littoral zone of a lake ecosystem. (This is the area close to shore.) This often results in direct mortality to turtles, frogs, muskrat and other wildlife that overwinter in the littoral zone of lakes. In Ston(e)y Lake, from Feb. 9 to March 20, 2020 water levels dropped below average levels. In Buckhorn Lake, including Chemong, water levels were drawn to minimum levels of 244.9 on March 6th.
In fact, the winter drawdown this year to minimum levels was well below average. A one-metre vertical drop in water levels exposes a significantly greater amount of shallow water littoral zone thereby resulting in a greater loss of fish and wildlife habitat and increased mortality rates of fish and wildlife including species at risk (e.g., Blanding’s turtles).
SPRING WATER FLUCTUATIONS ENDANGER WALLEYE SPAWNING
Walleye populations have been declining and are subject to multiple pressures including extreme water level fluctuations during spring spawning and incubation.
Walleye spawning occurs between 3-10 celsius with peak spawning usually around 7 celsius. “Fisheries and Oceans Canada Restricted Activity Timing Windows for the Protection of Fish and Fish Habitat” restrict in-water work in Southern Region between March 15 to May 31. The purpose of the restricted activity in these timing windows is for the protection of spawning fish, developing eggs and fry.
This year, spring water levels fluctuated between minimum levels of 233.6 on March 15th to 234.2 on April 4th then declined to 234.0 on April 12, then increased to 234.4 on April 22 then dropped again to 234.1 on April 27th. This extreme fluctuation during this period, including drawdown after walleye have spawned causes walleye egg mortality and a loss of larval walleye habitat. In addition, climate change is causing significant heat waves in early spring, as we are currently experiencing, which when combined with drawdown during spawning and incubation reduces the window for optimal spawning (7.7 c) and egg development (12.2 c) (Hasnain, S., Key Ecological Temperature Metrics for Canadian Freshwater Fishes, OMNR Applied Research and Development Branch, 2010).