Understanding Mazinaw Lake Dam
The Mazinaw Lake Dam is used for water control purposes and is located at the south end of Mazinaw Lake, which flows into Marble Lake. The Mazinaw Lake Dam is a concrete structure consisting of two sluices each containing seven 0.25 m x 0.30 m x 3.95 m stoplogs. An emergency bypass channel, which is at an elevation of 268.20 m acts as the access to the dam. The dam is owned and operated by MVC and the removal and replacement of stoplogs is done by a local contractor.
Lake trout typically spawn in mid-October to early November. The drawdown on Mazinaw Lake occurs throughout November and December with normal winter levels typically achieved in January. The current operating regime (in place and documented for more than 50 years) exposes known spawning shoals after the end of the spawning period and results in some egg mortality.
In the early 1990s, there was a proposal to begin the drawdown prior to the onset of lake trout spawning thereby ensuring that spawning would take place in areas that would not subsequently be dewatered. However, the proposed change to the operating regime required approval by Canadian Coast Guard under the provisions of the Navigable Waters Protection Act because the lower water levels would interfere with navigation on the lake. Although fish habitat management staff from the Department of Fisheries and Oceans supported the proposed change, there was some evidence that the lake continues to support a self-sustaining population of lake trout despite the late fall drawdown. Since it could not be demonstrated that the proposed change to the operating regime was critical to the sustainability of the lake trout, the Coast Guard denied approval of the proposed change to the operating regime.
Following the reconstruction of the dam in 1992, the operating plan for the structure was formally ratified between the Department of Fisheries and Oceans and the Coast Guard and a decision was made that no change to the timing of the drawdown would take place. The historical records (since the 1950’s) indicate that while stoplogs may have been removed from the dam in September or October to compensate for fall rains and to maintain stable lake levels, the drawdown of the lake itself has never occurred prior to November. No option is being considered to lower water levels, which would impact navigation or boat access only properties, in the summer on this lake. In maintaining the November-December drawdown, hunters will continue to have easier access during the fall hunt.
Subsequently, MNR has determined that Mazinaw Lake continues to support a self-sustaining population of lake trout within the current regime of water levels. Although the late fall drawdown undoubtedly affects lake trout, which spawn on the known shallow-water shoals, these recent findings support the theory of deep-spawning lake trout in Mazinaw Lake. Natural reproduction seems to be sustaining the lake trout population in Mazinaw Lake and, as a result, there is no need to revisit the option of an earlier drawdown to accommodate lake trout.
Seasonal Management Strategies
This dam is not normally operated in the spring until levels have stabilized from runoff.
Stoplogs are replaced to either maintain or bring lake levels up to the summer targets while maintaining adequate flow for walleye spawn below the dam.
Lake levels are targeted between 267.60 m and 267.90 m throughout the summer months (and until mid November) with a minimal flow being passed through the dam to keep water in the downstream channel.
The fall drawdown does not occur until after the deer hunting season, which is in mid-November. This ensures adequate water in the lake to allow navigation through the narrows, between the upper and lower lakes, as well as access to the east-shore residences.
Lake levels are targeted at its minimum level by mid-January at 266.70 m.
Eight (8) of the total fourteen (14) stoplogs in the dam are removed between mid-November and mid-December.