Glacial River Warren
Glacial River Warren or River Warren was a prehistoric river that drained Lake Agassiz in central North America between 11,700 and 9,400 years ago. The enormous outflow from this lake carved a mighty valley now occupied by the much smaller Minnesota River and the Upper Mississippi River. A part of the uppermost portion of the river channel was designated a National Natural Landmark in 1966.
Lake Agassiz was formed from the meltwaters of the Laurentide Ice Sheet during the Wisconsonian glaciation of the last ice age. Agassiz was an enormous body of water, up to 600–700 ft (180–210 m) deep, and at various times covering areas totaling over 110,000 sq mi (280,000 km2). Blocked by an ice sheet to the north, the lake water rose until about 9,700 years Before Present (BP), when it overtopped the Big Stone Moraine, a ridge of glacial drift left by the receding glacier, at the location of Browns Valley, Minnesota. The lake's outflow was catastrophic, and carved a gorge through the moraine 1 mi (1.6 km) wide and 130 ft (40 m) deep, which is now known as the Traverse Gap. The channel through the moraine, between Lake Traverse and Big Stone Lake, is now crossed by the divide between the watersheds of the Gulf of Mexico and Hudson Bay. Its significance was recognized by designation as a National Natural Landmark under the Historic Sites Act.
From Traverse Gap issued Glacial River Warren. From its inception until final abandonment of Agassiz' southern outlet, this stream drained the meltwater of that lake to the Mississippi valley. The drainage was not continuous, as Lake Agassiz periodically had other outlets. The Laurentide ice sheet retreated and advanced with climatic variations and these changes in ice cover contributed to isostatic adjustments in the level of the land over which the watercourses ran. These changes in turn uncovered or blocked the lake's other outlets to the sea.
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