Although some portions of the Winnipeg River basin have received normal precipitation amounts over the last few weeks, inflow to Lake of the Woods remains in the low-normal range. As per the LWCB Regulation Strategy, the objective heading into the fall will be to maintain stable water levels. With water loss through evaporation expected to increase over the next month, this may require outflow reductions. An initial outflow reduction is scheduled for Tuesday, September 2nd.
Why is lake evaporation typically higher in the fall?
There are three main drivers for evaporation: (1) warm water temperatures coupled with cold air temperatures, (2) low relative humidity and (3) high wind speeds. In summer, the air temperature is higher than the water temperature and humidity also tends to be high. But by September, these conditions reverse. The water has warmed and its temperature can be higher than the air temperature. Humidity drops and wind speeds can increase. At Lake of the Woods, it is common in September for evaporation to remove more water from the lake than it receives from rainfall and tributaries. This means water levels would drop even if no water was flowing out to the Winnipeg River.
The current level of Lake of the Woods 322.76 m (1058.9 ft) a 15th percentile level for this time of year. The average lake level declined by 3 cm (1 in) over the past week and is expected to remain stable over the next week. Lake of the Woods authorized outflow will decrease to 200 m3/s on Tuesday, September 2nd.
This outflow change will cause levels along to Winnipeg River to decrease by 9 cm (3.5 in) below the Norman Dam, by 6 cm (2.5 in) above Kimberley Rapids and by 3 cm (1 in) at the Dalles. This outflow reduction will have no impact on levels at Minaki.
The current level of Lac Seul is 356.24 m (1168.8 ft), a 25th percentile level, within the normal range for this time of year. The lake level increased 3 cm (1 in) over the last week and is expected to increase by 2 to 5 cm (0.75 to 2 in) over the next week. Lac Seul authorized outflow is 125 m3/s with no changes scheduled at this time.