Heidi Shaffer, Published June 27 2011
Souris vs. Red: What’s the difference in our floods?
What’s happening this week in Minot is far beyond the type of destruction Fargo saw during its 2009 crest, but what makes these record floods different?
So far, several factors in Minot present a contrast to the kinds of flooding we’re used to seeing in the Fargo-Moorhead area:
The Souris is controlled by a dam structure just north of the city – making the river rise faster – the source of the runoff is rainfall, not snowmelt, and the elevation of the surrounding land rises faster and higher.
Fargo’s flat. Minot is not.
The Red River’s true valley is only a couple hundred yards wide. The larger area surrounding Fargo – what’s commonly referred to as the Red River Valley – is actually the bottom of glacial Lake Agassiz that spreads for dozens of miles to the east and west.
In the Red River Valley, the elevation varies by about 10 feet from north to south Fargo and about 10 feet from the Red River past West Fargo.
Minot’s river channel is far more defined. The older parts of Minot sit in a valley, with the rest of the city perched on high ground.
Elevation in some areas along the Souris River valley rises about 200 feet from the center of the channel in about a mile in both directions, according to a cross-section provided by the U.S. Army Corps of Engineers.
Minot’s higher elevation saves portions of the city, but the Souris meanders in and out of city blocks, making it harder to protect, said Tim Bertschi, the area corps engineer who has helped orchestrate flood fights in both Fargo and Minot.
“It’s hard to defend to high levels because the river snakes through backyards,” Bertschi said.
Minot experienced a crest of 1,561.72 feet at 11 p.m. Saturday, about 3.5 feet higher than their record flood of 1881.
While the flood this past week pushed about a quarter of the city’s 41,000 people out of their homes, the rest of the city remains on high ground, said Allen Schlag, a hydrologist with the National Weather Service in Bismarck.
Without any flood protection, the city of Fargo is under water at about 43.5 feet, Fargo City Engineer Mark Bittner said. That’s less than 3 feet higher than the 2009 record of 40.84 feet.
“When it gets that high, if West Fargo doesn’t do something, it’s overtopping West Fargo,” Bittner said. “A 4-foot rise is miles and miles here.”
Conversely, the flat landscape of the Red River Valley allows the floodwaters more area to spread, meaning it takes more and more water to create a rise in the river level.
Snowmelt vs. rainfall
The Souris River Valley received above normal amounts of snowfall this winter, but it’s the unprecedented amount of rain this spring that is causing record flooding, Schlag said.
“It’s really difficult to get a grasp of the enormity and the rarity of this kind of water in the Souris River Basin, and it’s been that way all spring,” he said.
The entire Souris River Valley in southern Saskatchewan and northwestern North Dakota received rainstorm after rainstorm starting in May and June, Schlag said.
The river system and dam structures didn’t have time to keep up because they were still at capacity from this spring’s heavy snowmelt, he said.
Fargo officials began preparations months in advance of this spring’s flood. Snowmelt allows more time for more accurate predictions and protection measures, said Bertschi of the corps.
The Souris River is controlled by three reservoirs in Canada and one – Lake Darling – just north of Minot.
Minot’s direct impact by dam releases is one of the main differences compared to Fargo, said Bertschi.
The dam wall at Lake Darling holds back about 50 to 60 feet of water, Schlag said. Once the structure is full, the corps must let the water through or face a tidal wave over the top of the reservoir, he said.
To create the same kind of dam in the Red River Valley, the structure would likely have to be many miles wide to hold back the same amount of water, Schlag said.
Minot’s other flood control is a series of levees built in the 1970s by the Army Corps of Engineers to protect against a 100-year flood.
The situation in Minot shows why a Red River diversion is still the best flood control option for Fargo, said Aaron Snyder, a corps project manager for the diversion.
“It’s a perfect example of why a storage-only solution is maybe not the best answer,” Snyder said.
Many opponents of the diversion have questioned why the corps doesn’t use storage and a system of levees to protect Fargo instead of a 36-mile channel that splits the Red in two during major floods.
Once levees are built to a certain level, they can’t go any higher and are overtopped, said Fargo Engineer Mark Bittner.
“On the Missouri and Souris (rivers), you can see that storage within the reservoir works really, really well until you don’t have any storage left,” Bittner said. “Once it’s filled up, whatever goes in has to come out.”
Readers can reach Forum reporter Heidi Shaffer at (701) 241-5511