Gardiner dam tops the list

Spring thaw isn’t on the minds of most Canadians yet, but water forecasters have been thinking about melting snow for months.

“Nov. 1 is called the start of the new water year because basically any precipitation that falls, for the most part, from (November) on in is going to be snow and won’t show up into the rivers until next year,” said Scott Vatcher, river flow forecaster with Alberta’s environment ministry.

November may seem early to begin gathering data on snow depths in the Rocky Mountains, but the precipitation data is crucial for government agencies that operate the hundreds of water reservoirs on the Prairies.

The primary operators of water reservoirs are Alberta Environment, the Saskatchewan Watershed Authority, Manitoba Water Stewardship and the federal government’s Agri-Environment Services Branch (AESB), formerly called the Prairie Farm Rehabilitation Administration.

The Alberta government owns 200 of the province’s 1,400 dams. Twenty-three of them are in southern Alberta and their reservoirs can store five million cubic decameters.

One cubic decameter is a standard unit to measure reservoir volume and is equivalent to one million litres.

The Saskatchewan Watershed Authority operates 44 reservoirs. The largest reservoirs – Gardiner, Rafferty, Alameda, Blackstrap North, Blackstrap South, Buffalo Pound and Katepwa – can hold back 10.4 cubic decametres of water.

The Gardiner Dam is the largest earth fill dam in Canada and has a reservoir capacity of 9.4 million cubic decametres. For comparison, the reservoir behind Hoover Dam, Lake Mead, is the largest in the United States and has a capacity of 35 million cubic decametres.

The AESB operate 33 reservoirs in Saskatchewan with a full supply level volume of 500,000 cubic decametres, said Rob Wiebe, manager of Agriculture Canada’s water infrastructure division.

Manitoba has 58 dams in its crop-growing regions. Those reservoirs, operated by the AESB and Manitoba Water Stewardship, have a full supply level of 641,000 cubic decametres.

Most of that water is in the Shellmouth Reservoir on the Assiniboine River near Russell. It has a full supply level of 480,000 cubic decametres.

Alf Warkentin, director of flood forecasting for Manitoba Water Stewardship, said detailed data on snow depths, snow water content and soil moisture is essential to manage reservoir water levels.

“We have to make sure that we have enough water in that reservoir so that we can provide adequate flows for irrigation and for municipal and industrial use,” he said, referring specifically to the Shellmouth Reservoir.

Wiebe said as a reservoir operator, the AESB has flexibility in maintaining water level, but the branch is expected to operate within a specified range, according to the Saskatchewan Watershed Authority.

“The province is the resource manager,” Wiebe said. “They understand and know what the reservoir capacities are and what the downstream demands are and the licensed uses.”

Flow in and flow out essentially determines how much water is in a reservoir, but siltation is also a factor.

Silt can reduce the storage capacity of a reservoir, but it depends on the river and the land it flows through. Silt buildup has reduced the capacity of a reservoir on the Milk River in Montana by 30 percent.

However, Wiebe said silt buildup isn’t a significant problem for AESB’s reservoirs in Saskatchewan.

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