A rendering of the Mississippi River and a walking path along it.

Among Giants: A Brief Hydrological History of the Upper Midwest

• December 20, 2024

The Mississippi River and its valley dominate the physical geography of the Twin Cities. It — along with the aptly named Minnesota River, its largest Minnesota tributary, and the Red River, which drains northwestern Minnesota and the eastern Dakotas on its slow course toward Hudson Bay — offers a window into the thousands of years of sometimes violent geological change that shaped the Upper Midwest we know today.

Sometimes trivia is just that: trivial. Some other times, though, it cracks the world open a bit, into something a little less disenchanted. Such is this trinket about my local stretch of the Mississippi River as it bends its way through St. Paul, and the giant from which it emerged.
As you might notice upon visiting the Twin Cities, the Mississippi’s valley can be rather narrow or strangely wide, depending on the part of town. (An informal survey of friends and colleagues revealed this is, in fact, quite often not noticed.)
To understand why, you might first look north and West, to the Red River Valley along the North Dakota border. The Red River flows north into Canada and, eventually, Hudson Bay, but “valley” is a real misnomer; locals believably claim this to be the flattest place on earth.
You also need to look to the past. Driving through the Red River Valley feels akin to being at sea, the earth and sky stretching to their fullest in all directions.* This is not entirely a coincidence; the land here is flat because it was once the bottom of a vast, shallow, ancient lake, Lake Agassiz, formed by melting glaciers. (*Or it feels depressingly dull. Mileage may vary)
Its size fluctuated over the millennia as glaciers to the north advanced and retreated, but at times it was much bigger than any existing lake on earth today. Beach Dunes built up around it, which along with glacial moraines (ridges comprised of gravel/debris from earlier glaciers) formed natural dikes. The lake’s intermittent retreats weren’t always gentle. About 11,000 years ago, the southernmost moraine burst in a violent flood, the Glacial River Warren forming to drain away the lake. The new river gouged a mile-wide channel in a matter of weeks, scouring down to some of the earth’s oldest exposed bedrock.
The magnitude can still be felt, though it’s easy to miss amidst today’s farms and prairie. For example: Brown’s Valley, MN, sits just downstream from where the river warren first erupted out of Lake Agassiz, the entire town (pop: 558) easily sitting within the glacial river channel.
Large remnant lakes also lie in Agassiz’s footprint, and echoes of the ancient lake resurface in the right/ wrong conditions: long, snowy winters followed by wet springs, months’ worth of snowmelt backing into the still-frozen Canadian prairie, flooding farmland and towns along the rivers.
The Minnesota River spills into the Mississippi River near the Twin Cities today as a slow-moving complex of wetlands, rivulets, and islands beneath towering (by Minnesotan standards) bluffs. Upstream, the Mississippi never partook in the River Warren’s cataclysmic drama, its valley consequently much narrower and (a well-informed St. Paulite might be tempted to say) much more boring. Downstream, entire neighborhoods now sit within the scoured valley carved by The glacial river, the Mississippi meandering within its former. The confluence, known as bdote, has long been sacred to the Dakota people. It also holds the U.S. government’s first colonizing outpost in MInnesota, Fort Snelling (1819), now in the shadow of a sprawling airport.
In its more dramatic past, a mile-wide waterfall formed along the glacial river, spanning the channel in St. Paul. anticipating future rivalries, the River Warren falls slowly retreated upstream into Minneapolis as its riverbed weathered away over several thousand years.
The River Warren can be easy to sleuth out on some maps. Once you're aware of it, it's easy to spot from the street, too. There’s a nice stretch of parkland below downtown St. Paul, on a shelf between two river channels: the Mississippi's immediately below, the river warren’s rising above. One end of the park sits under the shadow of the “high bridge,” an accidental mimic of the river once spanning this gorge, carved within weeks by riverine fury. I think about the glacial river often when there, my kids and patient friends reminding me that yes, I’ve mentioned it to them before. But I can’t help myself; we live among giants, in shadows cast by their absence.
© Richard Bohannon, 2022. Map Data from Natural Earth, MNTOPO, MNDOT, Minnesota DNR, and commission for Environmental cooperation. Lake Agassiz shorelines based on Richard W. Ojakangas and Charles l. Matsch, Minnesota’s Geology (Univ. of Minnesota press, 1982) and H.E. Wright. “Geologic History of Minnesota Rivers,” Educational series 7 (Univ. of Minnesota Press, 1990). I am very much not a geologist, so any errors are also very much my own.

Rick Bohannon

About Rick Bohannon

Pronouns: he/him

Rick Bohannon is a cartographer and sociologist, and teaches in the College of Individualized Studies at Metro State University in St. Paul.