If asked to define the term continental divide, most North Americans would describe a long and lofty mountain range whose crest marks a dividing line for runoff of precipitation. That would be the roughly north-south chain of mountains across New Mexico, Colorado, Wyoming, Idaho and Montana, plus the extension of this terrain southward into Mexico and northward into Canada and Alaska. Much of this continental divide rises to such high elevations that it is visible from great distances. There is nothing subtle about it. It is often referred to as The Continental Divide, as though it is the one and only in our continent. The resulting image is that if a single drop of rainfall splits in half across the crest, the west droplet will end up in the Pacific Ocean while the east droplet will make its way to the Atlantic Ocean.
Continental Divides and Drainage Basins
What happens with water draining off the east of this mountain-chain continental divide is not simple. Yes, that water does not drain directly into the Pacific Ocean (although all of Earth’s oceans are connected). But it also does not simply drain directly into the Atlantic Ocean. Some finds its way to the Gulf of Mexico, some to the Arctic Ocean, some to Hudson Bay, and the rest goes directly into the Atlantic Ocean. Each of these places where flowing surface water leaves the North American Continent is an outlet for a geographic area with an integrated system of streams and rivers — an area called a drainage basin. Continental Divides are the boundaries between drainage basins. Many of North America’s continental divides are not mountainous. Some are in nearly flat country of the Great Plains. So if you happen to be driving Interstate 94 between Valley City and Jamestown, North Dakota, and see a large sign that announces “Continental Divide: Elevation 1490 Feet”, that message is true … not a practical joke.
A Hidden Continental Divide in Minnesota
In west central Minnesota, part of a continental divide is “hidden” and essentially invisible to the naked eye. There is no mountain crest to define this drainage boundary. Instead, it lies on a valley floor, 130 feet below surrounding flat terrain. At a certain elevation across that valley floor, half of the mythical split raindrop flows northward and eventually into Hudson Bay whereas the other half flows southward into the Gulf of Mexico. The divide is invisible to the naked eye, because only an extremely careful and accurate land-elevation survey can locate where precipitation runoff splits into north and south directions. This subtle stretch of continental divide is in Browns Valley, Minnesota, my hometown. And it’s part of the same continental divide that the traveler crosses along I-94 between Valley City and Jamestown, North Dakota.
Welcome to the Valley of Glacial River Warren
The Quaternary Period of the geological time scale encompasses the most recent two million years. Quaternary is subdivided into Pleistocene (two million to ten thousand years ago) and Holocene (ten thousand years ago to the present). Glaciers advanced southward from the Arctic repeatedly during Pleistocene time, covering most of present day Canada and much of northern USA. The most recent of these ice sheets covered Minnesota and lots of adjacent terrain. When this ice sheet began to stagnate, about thirteen thousand years ago, and then retreat northward in response to a warming climate, melt water created a lake (Agassiz) that was contained by glacial deposits of sand and gravel and boulders (collectively called moraines) on the south and by the retreating front of the glacier on the north. The lake grew to cover part of northwestern Minnesota, part of the eastern Dakotas, and much of Manitoba Province of Canada. As Lake Agassiz increased in surface area and depth, it eventually overflowed its southern boundary, draining vast amounts of water that eroded the valley in which the town of Browns Valley is located. Further downstream, that Pleistocene river, named Glacial River Warren, created the valley in which today’s Minnesota River flows, and it also eroded out much of the valley of the Mississippi River downstream from the confluence of the Minnesota and Mississippi Rivers at Minneapolis/Saint Paul.
A New Continental Divide is Born
Eventually, the front of the melting ice sheet retreated far enough to allow the northward flow of Lake Agassiz’s water into Hudson Bay. That effectively drained Lake Agassiz and that’s when a new continental divide was born, right across where Browns Valley, Minnesota, sits today. The part of the valley through which so much water had surged southward, creating the paths of today’s Minnesota and Mississippi Rivers, became the home of two long shallow lakes separated by a subtle continental divide. Part of the mythical drop of precipitation that is split in two at the divide flows northward into the Arctic Ocean by way of Lake Traverse and the Red River of the North (these two are the boundary between the Dakotas and Minnesota) and Hudson Bay. The other part of the mythical drop flows into Big Stone Lake, the Minnesota River, and then the Mississippi River on its way to the Gulf of Mexico. The town of Browns Valley sits between and barely above the two shallow lakes.
Growing up on a Continental Divide
In hindsight, I’m pretty sure the “hidden” north-south continental divide runs right under the house where I grew up. We lived in the lowest part of town. As a young kid I wasn’t thinking about such technical topics. But now, as a PhD geologist with decades of professional experience under my belt, I realize that my hometown is located at an extremely interesting geological spot, made unique by its history of Pleistocene glaciations. The first-time visitor who drives in from the east or west today is surprised by the valley, having arrived over miles and miles of pretty flat ground to get there. But once at its rim looking in, one sees a valley about a mile wide, one hundred thirty feet deep, and stretching northeast and southeast as far as one can see from the highway access. The town sits at the change in valley trend, about a hundred yards from South Dakota. Part of Browns Valley is built on an ancient gravel bar created by Glacial River Warren. The rest of the town is somewhat lower, on the delta of the Little Minnesota River, which enters the valley from the west, winds its way through town, and feeds into Big Stone Lake. Throughout its 150-year history to date, the parts of Browns Valley on the delta have been repeatedly flooded by spring snowmelt and runoff. Lake Traverse and Big Stone Lake merge until their sluggish outflow in opposite directions exposes the temporarily submerged continental divide. The most recent serious springtime flood (March of 2007) was so damaging to many houses that a canal has been built to carry flood water of the Little Minnesota River directly to Big Stone Lake, bypassing town. Presumably, the stretch of continental divide in Browns Valley will never again be flooded.
Sources of information:
Mark Gonzalez, 2003, "Continental Divides in North Dakota and North America" North Dakota Geological Society Newsletter, v. 30, no. 1, 7 pages.
Richard Ojakangas and Charles L. Matsch, 1982, Minnesota’s Geology: University of Minnesota Press, Minneapolis, 255 pages.
J. T. Teller and Lee Clayton editors, 1983, "Glacial Lake Agassiz" Geological Association of Canada Special Paper 26, 451 pages.
Wendell Duffield - I'm a PhD geologist who has studied volcanoes here and there around the world for a bit more than four decades. I retired from the U.S. ...
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