The Red River Valley, glacial Lake Agassiz. For many among us, that probably brings to mind rich farmland and perhaps a finger shaped lake from the end of the last ice age that covered the area along North Dakota/Minnesota border. It would have been fifty miles wide or so, with the southern end in bit south of where North and South Dakota meet Minnesota. The northern end would have been, well, not sure. At the margin of the glacier, somewhere up by Winnipeg, maybe.
Glacial Lake Agassiz, as the name implies, formed as the glacier retreated toward the end of the last ice age. Depressions filled to become ponds, then lakes, and some drained into others. All of course on a gargantuan scale. The areal coverage of glacial Lake Agassiz changed markedly over a span of 11-12,000 years or so (roughly 20,000-9,000 YPB). But if you were to look at a map of the area covered at some point by the lake it is mind boggling. It covered parts of Saskatchewan, Manitoba, and Ontario, along with parts of North Dakota, South Dakota, and Minnesota. It covered some 300,000 to 400,000 square miles, but again, not all at once.
In the Red River Valley, it existed from around 12,000-9,000 YPB. The water level varied considerably, forming beachlines at different elevations. The beachlines are much more noticeable on the east or Minnesota side. That is because the wave action resulting from the prevailing west winds cut deep into the shoreline, and also delivered coarser sediments and washed away the finer sediments. Driving highway 10 eastward from Fargo/Moorhead, the old beachlines are quite noticeable in places. Contrast that with driving west of Fargo/Moorhead on I-94. They are hard to detect. But look closely, for example, around the Buffalo/Alice exit next time you travel that way.
Somewhere around 10,000 years ago, glacial Lake Agassiz overcame its bank on the southern end. A colossal outpouring and downcutting commenced, flowing southeastward, from near what is now Browns Valley, MN. This was the formation of glacial River Warren. It cut an impressive valley, all the way down to Mankato, then northward to Minneapolis/St. Paul. Today the Minnesota River flows through that valley and of course we know it as the Minnesota River Valley. But the water from glacial Lake Agassiz and the River Warren did all the hard work!
~Chuck Lura
