Saturated ground growing concern for spring runoff
Effect on spring runoff
Minot and Souris Valley residents weary of the record rainfall of recent weeks know full well that saturated ground heading into freeze-up can lead to serious implications next spring.
Should the spring melt begin over soggy ground, little to none of the snowpack would be absorbed. It’s not a good scenario but it is only one piece of the runoff puzzle.
“Soil conditions really is just one of three major components that go into our spring flood runoff forecast,” said Allen Schlag, National Weather Service hydrologist in Bismarck. “Second is how much snow we get and how much water is in it. What the melt period looks like is by far the most important factor and we won’t have any idea on that until February or March.”
The amount of snowfall the region will receive this winter and how much moisture it will contain is impossible to know at this early date. The long-range forecasters at the National Oceanic and Atmospheric Administration’s Climate Prediction Center take an educated guess at it though. The three-month outlook issued Sept. 19 by the CPC, covering October through December, rates the possibility of above normal precipitation through those months at 33 percent for most of North Dakota and the Souris River basin.
October and November can actually be months when the soil moisture content decreases. However, this year’s saturated soil might already be too much to overcome this late in the year.
“It’s more and more difficult to imagine us drying out,” said Schlag. “The near term forecast is relatively dry after this spell, but the temperature range is not one to lead to rapid drying out of the soils. Even with normal moisture going forward we’ll have more than normal moisture at freeze-up.”
Schlag said he was “fairly concerned” about the soggy soil conditions dominating the state. He also noted that wetlands, the potholes that are normally dry or almost dry at this time of year, are full or nearly full of water. Most years potholes have ample room to store spring snowmelt runoff but, if conditions persist, those extra water storage areas will overflow quickly come spring.
Minot is unique in that it has four major reservoirs regulating flows on the Souris River above the city. They are Boundary, Rafferty and Grant Devine Reservoirs all in Saskatchewan, and Lake Darling about 25 miles northwest of the city. All of those reservoirs can play important roles in handling spring runoff.
Boundary Reservoir is filled by water flowing in Long Creek and is connected via an overflow channel to Rafferty. Rafferty’s primary water source is the Souris River in Saskatchewan although several smaller drainages also lead into what is the largest impoundment on the Souris. Grant Devine, formerly known as Alameda, is situated on Moose Mountain Creek just above its junction with the Souris.
Lake Darling is the last impoundment on the Souris River above Minot. With a maximum capacity of just over 92,000 acre feet of water it compares in size to Grant Devine, which has a maximum storage of slightly more than 85,000 acre feet. Those two reservoirs however, are dwarfed in size by Rafferty which has a capacity of more than 356,000 acre feet of water.
Water levels in the reservoirs is regulated by the International Joint Agreement between the United States and Canada. All three reservoirs have designated levels they must be drawn down to by Feb. 1 each year, a date selected because it is far in front of the anticipated annual melt.
As of Wednesday the water levels of the three above mentioned impoundments were already at or very near their Feb. 1 International Agreement requirements. Some water that has been flowing over control structures in Minot but that is not as a result of increased releases from Lake Darling Dam. It is from local runoff.
“You’ve got a pretty active Souris River running through Minot right now,” noted Schlag on Tuesday.
The Des Lacs River, which flows unchecked from Kenmare to its junction with the Souris at Burlington, is responsible for much of the recent movement in the Souris within the city. The Des Lacs increased from a flow of less than 100 cubic feet per second to 600 cfs due to late September rainfall. Increased flows in the Des Lacs are often short lived though. The river was declining earlier this week following a rise of nearly four feet. The Des Lacs stayed within its banks.
The Souris River at the Boy Scout Bridge immediately west of Minot, buoyed by additional flows from the Des Lacs, jumped from 100 cfs to 900 cfs following the most recent rainfall, rising from approximately 4.5 feet to 8 feet but not threatening to leave its banks.
The Souris River at Sherwood rose only about a quarter inch due to late September rains. Rises in the Souris downstream from Minot have been evident in recent days but flows remained within the river’s banks.