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Glacial River Warren or River Warren was a prehistoric river that drained Lake Agassizmarker 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.


Lake Agassiz was formed from the meltwaters of the Laurentide Ice Sheet during the Wisconsonian glaciation of the last ice age. Agassiz was a huge body of water, up to 600-700 feet (~200 m) deep, and at various times covering areas totaling over 110,000 square miles (~300,000 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, Minnesotamarker. The lake's outflow was catastrophic at times, and carved a gorge through the moraine a mile (1.6 km) wide and 130 feet (~40 m) deep, which is now known as the Traverse Gapmarker.

From the gap issued the 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 however 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.


While active this turbulent stream cut and eroded a bed up to five miles (8 km) wide and 250 feet (80 m) deep. This has left a valley which starts at Traverse Gap near Browns Valley, Minnesotamarker, goes southeast to Mankatomarker, then turns northeast to the Twin Citiesmarker. River Warren was joined by the comparatively small Mississippimarker at Fort Snellingmarker, from which the valley continues northeast to present-day Saint Paulmarker, where the massive River Warren Falls once graced the landscape. Over 1700 years this waterfall retreated upstream and undercut the Mississippi at the site of Fort Snellingmarker. The falls then split. The Mississippi falls migrated upstream to form Saint Anthony Fallsmarker and create Minnehaha Fallsmarker in Minneapolismarker. The River Warren falls receded west in the Minnesota River valley until they reached an older buried river valley about two miles (3 km) west of the confluence, where the falls were extinguished.

From Saint Paul the great valley goes southeast to Prescott, Wisconsinmarker, where it is joined by the St. Croix River, itself once the outlet of another proglacial lake, Glacial Lake Duluth which occupied the western part of Lake Superiormarker. From its confluence with the St. Croix the valley continues southeast along the Minnesota-Wisconsin border. River Warren's effects include the creation of bluffs along the valleys of the Minnesota and Mississippi Rivers, and contributed to the formation of Lake Pepinmarker.


By about 9400 BP, the ice sheet finally retreated sufficiently far to the north that Lake Agassiz permanently took another outlet and receded below the level of Traverse Gap. River Warren then ceased to run. The Lake Agassiz area watershed now feeds the Red River of the Northmarker which flows north, ultimately to Hudson Baymarker. River Warren's upper valley in the Traverse Gap is now occupied by the tiny Little Minnesota Rivermarker, which flows into Big Stone Lakemarker and the Minnesota River, which follows the greater river's ancient bed to its confluence with the Mississippi River. These streams occupy only a small cross-section of River Warren's riverbed.


The hydrology of the oversized valley was first explained by General G. K. Warren in 1868. He made a detailed survey of the valley in his search for possible transcontinental railroad routes. Posthumously, in appreciation of this work, the glacial river that was the outlet of Lake Agassiz was named River Warren.

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