| Summary: | The youngest and narrowest segment of the Missouri River valley in North Dakota is in the New Town area between Four Bears Bridge and Van Hook Bay. As recently as 14,000 years ago, a glacier forced the river, which had been flowing through a broad valley now known as the "Van Hook Arm," into a new position a few miles farther west. There are a few exceptions to the "wide east-west valleys/narrow north-south valleys" pattern. At Bismarck-Mandan the Missouri River valley is only about two miles wide where Interstate Highway 94 crosses the river, a fairly narrow valley. However, on the south side of Bismarck the valley broadens to about six miles wide, although it retains its steep western edge. The reason for this widening is that, prior to glaciation, the Heart and Little Heart Rivers, which today flow into the Missouri River (the Heart River enters the Missouri River at Mandan; the Little Heart enters about 10 miles south of Mandan), converged a few miles east of Bismarck. The old, combined Heart-Little Heart valley still exists as the broad lowland south of Bismarck � a wide spot in the Missouri River valley. The Missouri River valley north of Bismarck-Mandan is much younger (formed during a glacial event, perhaps 25,000 years ago) than is the valley several miles south of the city, where it coincides with � crosses � the old, preglacial northeast-trending Heart-Little Heart valley. The modern route of the river is only the most recent of many routes that the earlier "Missouri" rivers followed through the state at various times during the Ice Age. What routes did these previous "Missouri" Rivers follow through North Dakota on their way to the Gulf of Mexico? All of them flowed generally southward because the original, northerly routes of all other rivers into Canada were blocked each time glaciers advanced into the state. In eastern North Dakota most of these early Missouri River valleys are buried today beneath thick layers of glacial sediment. One of them had a route that took it southward past Cooperstown and Valley City to the southeastern corner of the state. In western North Dakota an early Missouri River was formed when a glacier advanced into the southwestern part of the state. A river flowed southeastward along the edge of the glacier, forming a valley from the area of the Killdeer Mountains, past Hebron and Glen UI]in, and on to the Fort Yates area. This valley is a large one that probably served as the "Missouri" River for a much longer period of time than has the modern Missouri River valley, at least up to now. Interstate Highway 94 crosses this old Missouri River valley about half way between Dickinson and Mandan. In summary, the modern Missouri River valley is a "composite" of old, wide preglacial valleys and younger, narrower valleys that were cut at various times during the Ice Age. The parts of the Missouri River valley that extend west-east are generally wider and these are the older segments. The parts that extend north-south are generally narrower and younger. Even though many other earlier versions of the Missouri River existed during the Ice Age in eastern North Dakota, most of them were later buried beneath thick deposits of glacial sediment. The modern Missouri River valley is simply the latest in a continuing series. After the next glacier has come and gone, the "new" Missouri River will undoubtedly follow a different route than it does today. 1999-2001 North Dakota Blue Book 479
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