Glaciation and Forming the Missouri River Trench

Glaciation and Forming the Missouri River Trench. . Glaciers altered river courses and molded the North Dakota landscape, creating the gently rolling, hilly terrain seen in most areas of the state today. Prior to about three million years ago, all the rivers in today's North Dakota flowed toward the northeast, as do today's modern Knife, Heart, and Cannonball rivers. These rivers, because they formed over millions of years, had broad valleys. None of them could be considered to be the "Missouri River."

During the most-recent Ice Age, a massive, one-to-two-mile-thick ice sheet blanketed North America. Its center was located west of Hudson Bay. Glaciers are not static blocks of ice and, during the last ice age, glaciers advanced and retreated many times as temperatures cooled and warmed. The glacial ice scoured and gouged landscapes, picked up, and then deposited large quantities of sediment. When the glaciers melted, they released large amounts of water, some of it gradually as small streams, some quickly, through newly-formed valleys into lakes and rivers.

About 25,000 years ago, glaciers flowing toward the south and west dammed rivers that had flowed northeast. A glacier east of today's Washburn dammed the Knife River. Water backed up along its banks, forming a reservoir lake. This lake eventually spilled over on its southeast corner, possibly in a spectacular flood. The water quickly carved a trench southward to today's Bismarck forming the segment of valley you see here. Similar events happened on many of western North Dakota's preglacial rivers. New diversion trenches directed meltwater to the south, eroding relatively deep valleys, and linking preglacial and glacial valleys together to form what we know today as the Missouri River. Those parts of the Missouri River that flow eastward are in wide, broad, older valleys-leftovers from river valleys that had carried preglacial drainage into Canada. River segments that flow south tend to be in narrower, steeper trenches. In a few places, such as the former river bed east of Washburn, the ancient wide valleys remain, but no streams flow in them.

Captions: , The drainage pattern in North Dakota prior to glaciation. Preglacial rivers flowed northeast into Hudson Bay. , The extent of glaciation in North Dakota. , The valley you see here was formed when an ice- dammed lake in the old Knife River Valley overflowed from a point near Washburn, carving the north- south segment to the Bismarck area. , The route of the preglacial McLean River. When the McLean River was blocked by a glacier, a proglacial lake formed to the west of the glacier. When the proglacial lake overflowed it eroded a narrow trench from Riverdale to past Stanton.