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Why Some Floods Last Longer

We’re getting used to more frequent floods in the Red River basin. But, now, in some parts of the basin, we’re also faced with flood waters staying around longer. They call it “duration.”

We can understand high waters at the peak of a flood. But it’s getting to be weeks now, and waters are still high in some areas. In the lower Sheyenne basin, for instance, water is still flowing over roads and some residents are still boating to get between their home and their cars. And, on the Assiniboine River in Manitoba, flooding is ongoing, requiring a controlled release of water through the Assiniboine River dikes.

So what accounts for longer-duration floods?

The answer in short is that a river basin or watershed can contain such a large volume of run-off that it takes a long time to drain it all.

If we use specifically the Sheyenne basin, we can see the elements that come into play that keep folks along the lower reaches of that river waiting – and waiting:

The winter snowpack in the Sheyenne basin was exceptional. And, like most of the Red River basin, it began the year with already saturated soils.

The Sheyenne tributary is large, second in size to the Assiniboine in Canada. The mere size of its drainage area makes it likely that the Sheyenne will have considerably more water to move than other tributaries.

The flow on the Sheyenne River is affected by two dams, the Baldhill Dam at Valley City and the Maple River Dam in rural Cass County. The good news is that the two dams reduce peak flows, thus protecting homes and roads at a flood’s peak. But the water that the dams store must be drawn down following that peak. That can take weeks, if not longer.

Waters break out of the Sheyenne at a number of points, most notably where the river turns from south to north for its final leg. While these breakouts provide relief to the channel, a good portion of those breakout waters later drain back into the Sheyenne, whether directly or via the Maple River, which enters the Sheyenne just a dozen miles or so before it enters the Red.

When the main stem Red River is also above flood stage, waters from the tributaries have to wait for those levels to fall before they can enter the channel. It’s a traffic jam of sorts.

Rainfall only adds to the problem. The second week of May has already produced four inches or more of precipitation in some areas of the Sheyenne basin. No matter how far away they fall, these waters will drain through the river’s full channel.

The situation along the Assiniboine River is very similar, with saturated soils, abundant winter snows and major precipitation events this spring, giving rise to a one-in-300-year flood with unprecedented flows.

The RRBC is a grassroots organization that is a chartered not-for-profit corporation under

the provisions of Manitoba, North Dakota, Minnesota, and

South Dakota law. Its website is: www.redriverbasincommission.org.

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