Longer Range Winnipeg River Basin Outlook
Throughout the spring, the LWCB has received requests for forecasts of the peak level for Lake of the Woods and areas along the Winnipeg and English Rivers. Because the levels of these water bodies are influenced by the amount of rainfall across the large watershed, it was not possible to provide peak estimates well in advance, as accurate forecasts of this rainfall are not available weeks ahead of time.
Now that Lake of the Woods appears to have crested, many are wondering how long it will take for a return to normal water levels on the lake and along the Winnipeg River downstream (as the lake drops, so does the outflow into the Winnipeg River).
As was the case while the lake was rising, it is not possible to develop a forecast for the rate of drop in the level over the long term because the rainfall is unknown. However, using some assumptions about upstream water supply and the hydraulics of the system, the Secretariat has developed reasonable scenarios for how the lake level drop might develop in the coming weeks and months. Scenarios are not forecasts, they are possible outcomes if certain assumptions are correct. A wide range of water level outcomes are still possible for the remainder of the summer and fall.
Current basin conditions – Lake of the Woods and Winnipeg River Sub-basin
Following the record precipitation across the Rainy-Lake of the Woods watershed in April and again in May, the levels of rivers and lakes around the region reached extremely high levels, many setting new records by a large margin. While many of the rivers in the system have returned to normal water levels and flows, Rainy Lake remains very high and the dam there remains fully opened (this dam is not under the jurisdiction of the LWCB). The higher the level of Rainy Lake is, the higher the outflow when the dam is fully opened. This has meant a long period of extremely high flow down the Rainy River into Lake of the Woods, even as smaller lakes and rivers around Kenora are returning to normal levels.
The large lakes and rivers are serving as the main collectors and conveyors of the record spring runoff, much of which is still making its way downstream through the watershed. This is a natural process that will take many weeks.
Outlook – Lake of the Woods and Winnipeg River
This flood can be thought of as a wave moving from upstream to downstream. The large lake furthest upstream, Lac la Croix, peaked first, followed by Namakan, then by Rainy Lake, and now Lake of the Woods. In each case, the drop after the peak is gradual but picks up pace. Lake of the Woods will follow the same trend, provided the weather remains favourable across the basin.
This process will take time, however, as Lake of the Woods is at the highest level since 1950. To return to the normal operating range, 323.39 m (1061.0 ft), will require a drop of 72 cm (2.4 ft) from today’s level (the top of the operating range is still 60 cm (2 ft) above the chart datum for Lake of the Woods).
Should a sustained period of hot and dry weather develop across the watershed, the level could reach the normal operating range in mid-August. A return to wet weather, however, would slow the drop in lake level. Extremely wet weather, as occurred September 2019, would see levels rising again, pushing back the return to the normal operating range into the late fall.
The dams in Kenora will remain fully opened until a level of at least 323.47 m (1061.25 ft) is reached. As the lake drops through the summer, the outflow will also fall, and the level of the Winnipeg River in Ontario will gradually drop. The level will drop more quickly once the lake is back within the normal operating range and larger outflow reductions can be made.
Current basin conditions – Lac Seul and English River Sub-basin
The English River basin also saw record April and May precipitation, leading to most river gauges across the region setting new records. These extreme flows peaked in most areas in June and have been dropping, but still remain high.
Unlike Lake of the Woods, Lac Seul is not constrained in its outflow and the lake level has not risen out of control. However, to maintain the lake level within the legislated limits in the face of record spring inflow, the LWCB directed very high outflows from Lac Seul. The combination of record flows from smaller tributaries and high flows out of Lac Seul resulted in record high water levels along the English River down to its confluence with the Winnipeg River near Boundary Falls, Ontario.
Outlook – Lac Seul and English River
Following generally drier weather in recent weeks, the LWCB has begun directing flow cuts at Lac Seul this week, with more planned next week. With favourable weather, the level of the main body of Lac Seul is expected to gradually fall over July while the level of Lost Lake at Hudson would fall to be closer to that of the main lake by late July.
Along the English River below Lac Seul, the combination of declining flows out of the natural tributaries and the flow cuts out of Lac Seul should see the river dropping more quickly through July. A drop of 60-90 cm (2-3 ft) in July is possible under dry or moderately dry weather conditions. However, a drop of 150 cm (5 ft) is needed to return to normal summer levels, and this is not likely before mid-August. A return to wet weather would slow the rate of decline, and extremely wet weather could see levels rising again this summer or fall.