Red River Flooding

Written by Amy Bauroth, volunteer

For those who live in the Red River Valley, it is our spring-time ritual. Consider the moisture content of the snow pack; note the moisture level of the soils from the previous fall; determine the frost depth; wish for well-timed days of above-freezing temperatures but freezing night-time temperatures; and, hope the rain holds off until after the snow melt has finished. Are these habits unique? Or is this just the way of life for any river community? Of course, every river basin has its own unique characteristics, but the river basin of the Red River of the North presents a challenging combination of attributes that can lead to major flooding scenarios as it flows north to Lake Winnipeg. This can partially be explained by looking at the history of the Red River Valley.

It is a misnomer to call the Red River Valley a valley since it is actually an ancient lake bed, located at what was once the southern tip of Lake Agassiz, a huge glacial lake, that covered much of today’s central Canada. The ground-up, fine sediment that washed off the Laurentide Glacier as it waxed and waned for thousands of years settled to the bottom of the lake bed, leaving the present-day clay soils of the Red River Valley. Once 300 feet deep, Lake Agassiz eventually broke through the natural dams holding it to the south. The result is our current landscape where the Red River of the North drains what is today the youngest and flattest major land feature in the contiguous United States. At a little over 9,000 years old, the Red is so young, geologically speaking, that it hasn’t had enough time to cut the sort of deep channels carved by older river systems.

The flatness of the landscape makes for meandering flows of water, sluggish drainage, and floods with large aerial expanses. Nowhere is the gradient steep, but the southern portion of the Red River has a slightly steeper gradient. According to North Dakota State University’s Fargo Geology Webpage (https://www.ndsu.edu/fargo_geology/whyflood.htm), a gradient of 5 inches per mile in the Fargo-Halstad area flattens to 1.5 inches per mile in the Drayton-Pembina area. In addition, the effect of deglaciation is still playing out in the region since the crust of the earth rebounds for thousands of years after a glacier retreats. Imagine something like geological memory foam returning to its original shape after a very heavy glacier sat on it. This rebound in the Red River Valley may only amount to a few millimeters per year and doesn’t have an important effect in the near term, but the process is happening at a more rapid pace in the northern portion of the valley, so the already low gradient between the origin of the Red in the south and Lake Winnipeg in the north is slowly becoming more level (see https://www.dmr.nd.gov/ndgs/ndnotes/Rebound/ for more information about glacial rebound in the area).

Some of the other troublesome characteristics of our floods are simply because the river flows to the north. The delicate timing of thawing, and water soaking into the ground (slow even when the ground is not frozen because of our clay soils) is further complicated by the fact that spring thaw generally begins earlier in the southern portion of the basin.  Ice jams are a common occurrence, and they create temporary dams causing flooding behind the blockage. Often, melt water from the south of the basin flowing to the north combines with more recently melted water or it is blocked by frozen conditions up north.  Frozen ground can also prolong the length of the flood when water is unable to soak into the soil.

Frequent flooding is inherent to the very nature of the Red River of the North and the ancient lake bed it drains. Each year, the timing and extent of precipitation and freeze patterns interact with the geological characteristics of the Red River Valley to present us with a guessing game of flood possibilities and leaving us to our spring-time rituals.

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