Forecasts indicated dry and sunny weather for the Winnipeg River basin over the next week. In the fall, these conditions lead to increased lake evaporation. The large lakes will be losing water through evaporation over the next week and as a result, mean lake levels are expected to decline gradually.
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.
Sustained, high winds in recent days are also causing water levels to fluctuate significantly and can temporarily lower local water levels further. The LWCB recommends that boaters exercise increased caution around potential hazards in shallow areas.
How Wind Affects Water Levels on Large Lakes
We often think of a lake’s surface as flat and still—aside from waves. But on large lakes like Lake of the Woods and Lac Seul, wind can cause noticeable changes in water levels from one location to another. This phenomenon is called wind setup. When sustained winds blow across the lake from one direction, they push water toward the far shore, raising the water level there while lowering it on the windward side. This creates a temporary tilt in the water surface. As wind speed, direction, and duration change, so does the distribution of water across the lake.
During windy periods, the average lake level computed from the measurements around the lake may change from day to day, even though the amount of water in the lake has changed little. And because lake inflow is computed using the change in lake level over time, this instability also causes inflow values to fluctuate from day to day.
The current level of Lake of the Woods is 322.70 m (1058.7 ft), a 15th percentile level, below the normal range for this time of year. The average lake level declined by 4 cm (1.5 in) over the past week and is expected to decrease by 1 to 2 cm (0.5 to 1 in) over the next week. Lake of the Woods authorized outflow is 150 m3/s with no changes scheduled.
The current level of Lac Seul is 356.39 m (1169.3 ft), a 50th percentile level, within the normal range for this time of year. The lake level increased by 3 cm (1 in) over the last week and is expected to decrease by 3 to 4 cm (1 to 2 in) over the next week. Lac Seul authorized outflow is 200 m3/s with no changes scheduled.