Rapid runoff likely: No major problems expected
This year’s snowmelt runoff is going to be a quick one. That’s the opinion of the National Weather Service which notes that it is unusually late in the season for spring runoff. Additionally, acknowledges the NWS, there are difficulties in forecasting a rapid runoff event.
“River forecast models have been hypersensitive to these conditions,” explained Allen Schlag, NWS hydrologist in Bismarck. “The big caveat to all of this is how open are the soils? We know the soils have been taking on water, but we’re not 100 percent certain how much extra water that will be produced and can’t go into the soil. I can say with confidence that, come Tuesday, you won’t be having any snowball fights up there.”
In other words, given the expectations for very warm weather, the NWS anticipates that the snowmelt will be rapid and complete in a very short period of time. During most runoff seasons the snowmelt would begin slowly and then build to a peak over several days or weeks. A normal melt allows forecasters to get an early preview of what is to follow as the melt progresses. That is not the case this year.
“Normally you get at least a couple of days to fine tune input to runoff models,” said Schlag. “With the rapid warmup we don’t have that luxury. In all honesty, we have a greater uncertainty in our forecast than we are usually accustomed to dealing with.”
A positive, said Schlag, is that no significant precipitation is expected in the next week to 10 days. Under the current conditions “significant” is defined as being greater than a half-inch of rain over a large portion of the drainage leading to the Souris River.
“It’s pretty rare that you see such a large coverage of snow with the onset of this warm of temperatures,” added Schlag, emphasizing once again the uniqueness of the combination of substantial snow cover and very warm temperatures.
No updated Flood Potential Outlook for the Souris River Basin has been issued by the NWS and, despite the lateness and expected speed of the runoff, no major flooding is expected. The NWS will continue to update expected flows at various forecast points.
The biggest move on the flow charts updated Friday was for the Des Lacs River at Foxholm. The Des Lacs was given a 95 percent chance of reaching 13 feet in the April 10 Flood Potential Outlook. Friday’s projection is that the Des Lacs will reach a level of 17 feet early Sunday afternoon. Flood stage on the Des Lacs at Foxholm is 16 feet but the river would need to top 19 feet before it is considered a threat to homes in Foxholm.
The Des Lacs was flowing gently through Burlington early Friday afternoon and remained well below the top of its banks. The Des Lacs joins the Souris at Burlington. The much larger Souris was very low at Burlington Friday afternoon.
Other drainages can influence flows in the Souris before the river reaches Minot. One major drainage is Gasman Coulee. That drainage was contributing very little to the flow in the Souris Friday. Several other minor drainages were flowing at only a trickle or not at all on Friday.
Conditions are expected to change this weekend. Significant rises in the Des Lacs and Souris River should become noticeable as early as this morning. However, says the NWS, the Souris is expected to remain contained through Minot regardless of the speed of the melt.
The level of the Souris is forecast to reach 15.7 feet at the Boy Scout Bridge northwest of the city late Sunday or early Monday. Moderate flood stage at that location is 17 feet. The Souris is forecast to reach 1,546.5 feet Sunday afternoon at Minot’s Broadway Bridge where flood stage is officially listed at 1,549 feet.
The release gates at Lake Darling Dam above Minot remained closed Friday and are not expected to be reopened until next Wednesday unless conditions warrant. The level of Lake Darling was 1,592.02 feet Friday afternoon. Overflow level for that reservoir is 1,601.8 feet.
Current plans call for an increase in releases from Lake Darling to reach a maximum of 2,000 cubic feet per second late next week as inflow from Canadian snowmelt is expected to begin reaching the reservoir. The flow at the Sherwood gauge, where the Souris enters North Dakota from neighboring Saskatchewan, was slightly over 100 cfs Friday.
Three reservoirs on the Souris in Saskatchewan have stopped releases after being drawn down to levels that are expected to be able to contain what is anticipated to be significant runoff. Eventually, outflow from Canadian reservoirs will be increased. The flows will be captured at Lake Darling so that a rise to a summer operating level of 1,597 feet can be achieved.
Of greatest concern at this time is that locations downstream from Minot are at an increased risk of flooding, primarily due to the fact that the Souris receives contributions from increased sources as it travels northward toward Manitoba. In addition, an abundant snow cover remained over much of the lower Souris basin this past week and is expected to add further runoff into the basin.
The Souris River at Logan is projected to reach as high as 35.7 feet later today. Flood stage at Logan is 34 feet. Major flood stage is expected farther downstream, at Towner, as early as Wednesday when the Souris is projected to reach 56 feet. If that occurs it would be the fifth highest level in the history of the Souris River at Towner.