Souris Basin runoff raised

The amount of water expected to enter the Souris River Basin from Canada has increased slightly from what was forecast in late January.

At that time the city of Minot was notified by Canadian authorities that a flow of 1,400 cubic feet per second could be expected in the Souris River where it enters North Dakota a few miles west of Sherwood. The latest calculations supplied by the Saskatchewan Water Authority now estimates the flow across the Canadian border into North Dakota will reach 1,651 cfs.

"What we've got coming across the border in terms of acre feet, we're talking about roughly 60,000 in a 30-day period," said Doug Johnson, hydrologist for the Saskatchewan Water Authority in Moose Jaw. "The Corps of Engineers and the National Weather Service are working with different forecasts than we have. We're differing in our forecasts a little bit."

The Souris River enters the northern end of Lake Darling on the Upper Souris National Wildlife Refuge. The 90-day volume of water forecast in the Souris at Sherwood is 121,600 acre feet. Lake Darling currently has an estimated 100,000 acre feet of storage. The flow in the Souris below Lake Darling is primarly determined by how much water is being released through flood control gates at Lake Darling Dam. Controlled releases during the runoff period will add additional storage capacity to the reservoir.

The Souris River channel through Minot is designed to handle a 100-year flood event of 5,000 cfs with 3 feet of freeboard remaining. City officials are closely monitoring runoff potential but, barring any major precipitation events prior to or during the melt, believe that spring runoff will remain in the river channel. The National Weather Service has been forecasting a 30 percent chance of the Souris River reaching a 100-year flood elevation of 1,549 feet at the Broadway gauge.

"I believe we could easily handle 7,500 cfs without doing a lot of stuff," said Alan Walter, public works director. "We might have some gates to close on some storm sewers in the city."

Walter added that he was awaiting a report from the National Weather Service in regard to spring flows that might occur in the Des Lacs River Basin. The Des Lacs is a free flowing tributary of the Souris with a drainage area of 939 square miles. It joins the Souris near Burlington and, given continual warm weather conditions, can empty within a few days.

Although it has rarely occurred, the Des Lacs River has the potential to reach flows in excess of 3,000 cfs. The National Weather Service projects that the Des Lacs River will reach 14 1/2 feet at Foxholm. Flood stage there is 16 feet.

Coincidentally, some of the largest water events on the Des Lacs occurred in years ending in "9." Record flooding occurred on the Des Lacs in 1939, 1949, 1969 and 1979. While the Des Lacs contributed a peak flow of 2,950 cfs into the Souris River during Minot's disastrous flood of 1969, the narrow river set its all-time record of 4,260 cfs in 1979.

The timing of the runoff is of critical importance and remains unpredictable. During serious flood events in Minot's past the peak flows of the Des Lacs and Souris Rivers arrived nearly simultaneously. However, comparisons to years past must take into account the storage capacity of two dams on the Souris River drainage in Canada. The Rafferty and Alameda Dams entered this past winter at very low levels and have the capacity to store an estimated 350,000 acre feet of runoff.

"What the Canadians are telling us is that Rafferty will rise 3 meters this spring instead of 2 and Alameda will go up about 4 meters," said Walter. "Boundary Lake is going to fill up this year and they'll use a diversion channel that connects Boundary to Rafferty Reservoir."

Johnson emphasizes that predictions of peak flows entering North Dakota are based on uncontrolled drainages below Rafferty and Alameda reservoirs.

"We'll store as much as we can and release water in coordination with North Dakota," explained Johnson. "Our goal is to not make anything worse at any place downstream. We feel there's going to be a large flow through Minot but I understand your flood protection is built to handle it."

Johnson noted that the snowpack in drainages leading into Lake Darling has the potential to add a significant amount of water and cautioned that the Des Lacs River, which drains approximately 100 square miles in Canada, could cause some trouble.

"The Des Lacs River Basin has a ton of snow and not a lot of storage," said Johnson. "As we move closer to the melt we'll release updated event forecasts."

Further information on projected river flows and runoff amounts calculated by the National Weather Servcie can be found at (weather.gov/bismarck). Canadian snowmelt forecasts can be found at (swa.ca).