Lake levels to reach post-regulation records this summer

Over the next four to six weeks, Lake Manitoba and Lake Winnipeg should reach heights unseen since the mid-20th century, before the province and Manitoba Hydro began regulating their levels.

Flooding across the Assiniboine and Red River drainage basins is expected to continue to drive up levels on Manitoba's big lakes until June and July, even though most major rivers have crested and are already receding.

On Lake Manitoba, the result will be the highest water since 1955, when the province's third-largest lake reached 816.3 feet above sea level. That year, just like this year, farms and towns along its edges were flooded out.

Lake Manitoba is expected to reach 815.8 feet by the middle of June, as the combined inflows on the Portage Diversion and the lake's two natural major tributaries -- the Waterhen River and Whitemud River -- dwarf what's flowing out at the Fairford River.

Roughly 47,000 cubic feet of water is flowing into Lake Manitoba every second, while only 15,000 cubic feet per second is flowing out. This surplus of water is expected to add another 0.9 feet to the lake's current level.

This level is not as worrisome as what happens when strong winds blow steadily from the north, creating a seiche -- an enormous standing wave created when water from one side of a lake is pushed into another.

Eventually, the Fairford River will let as much as 18,000 cfs out of Lake Manitoba. This will create more problems downstream at Lake St. Martin, where outflows on the Dauphin River do not exceed 12,000 cfs. Rising water at Lake St. Martin has already forced hundreds of people at the First Nation community of the same name to evacuate.

Dauphin River, meanwhile, drains into Lake Winnipeg, which has a greater capacity to handle floods. Lake Winnipeg is projected to rise another 0.2 feet by early July, when it's expected to max out at 716.6 feet.

That level will be Lake Winnipeg's highest level since 1974, when a record flood of 718.2 feet inundated Gimli and other lakeshore communities. The lake also crept above 716 feet in 2005, when heavy rainfall created a rare summer flood along the Red River.

Unlike Lake Manitoba, Lake Winnipeg's level is regulated by Manitoba Hydro, which uses five spillways and a turbine channel at the Jenpeg generating station to control part of the lake's outflow along the Nelson River.

As per its environmental license, Manitoba Hydro began allowing as much water as possible through the Jenpeg station last July and continued to do so over the winter, utilizing Two Mile Channel, a 30-foot trench at the north end of Lake Winnipeg that allows water to pass through even when ice reduces the capacity of the shallower natural outlet at Warren Landing.

Manitoba Hydro division manager David Cormie said it's a misconception Jenpeg drives up Lake Winnipeg's water levels, as the generating station and its channels have cut the range of water levels on the province's largest lake in half.

Before Jenpeg was completed, Lake Winnipeg levels fluctuated almost nine feet. Since Jenpeg, the variability has been just under five feet.

What's good for people and Manitoba Hydro, however, is not beneficial for the lake's environmental health, argues University of Manitoba aquatic ecologist Gordon Goldsborough.

Lakes need low-water years to expose the marsh mud to the air, which allows seeds to germinate and more vegetation to grow. High water years push the vegetation back -- but more than three feet of water will drown the marshes that scrub excessive nutrients from lakes, Goldsborough said.

"As long as you get a cycle of up and down, you have a balance of open water and vegetation," he said. "One of the concerns (about regulation) is the marshes on the big lakes are not getting enough low periods."

bartley.kives@freepress.mb.ca

Lake Manitoba water levels

Desired range: 810.9 to 812.9 feet above sea level

Level on May 20: 814.9

Projected peak (mid-June): 815.8 feet

Record peak (1955): 816.3

Record low (1943): 809.9

Lake Manitoba inflows this week

Portage Diversion: 33,000 cubic feet per second

Waterhen River: 11,750 cfs

Whitemud River: 2,500 cfs

TOTAL INFLOWS: 47,250 cfs

Outflow (Fairford/Dauphin River): 15,250 cfs

Lake Winnipeg levels

Desired range: 711 to 715 feet above sea level

Level on May 20: 716.4

Projected peak (early July): 716.6 feet

Record peak (1974): 718.2

Record low (1941): 709.4

Lake Winnipeg inflows this week

Red River: 100,000 cubic feet per second

Winnipeg River: 65,000 cfs

Saskatchewan River: 37,000 cfs

Other east-side rivers: 25,000 cfs

Fairford/Dauphin River: 15,000 cfs

Other west-side rivers: 8,000 cfs

TOTAL INFLOWS: 250,000 cfs

Outflow (Nelson River): 155,000 cfs

-- Manitoba Water Stewardship, Water Survey of Canada and Manitoba Hydro

Republished from the Winnipeg Free Press print edition May 21, 2011 A5