THE PRE-GLACIAL PLIOCENE MISSISSIPPI RIVER DRAINAGE DIVIDE IN CANADA

Recent research has placed the northern boundary of the pre-glacial Pliocene Mississippi River drainage basin in Manitoba and western Ontario, Canada. This study was undertaken to test this hypothesis. We restored the pre-glacial Pliocene topography of Manitoba, western Ontario, North Dakota, Minnesota, and western Wisconsin. Northeast and north-trending topographic and bedrock topographic profiles were tilted by raising the northern ends of the profiles up to 120 m along the southern margin of Hudson Bay to restore the remaining glacial isostatic rebound to approximate Pliocene pre-glacial elevations. In doing so, a clear flexure in the topographic profiles is apparent. The axis of the topographic flexure trends southeast across Manitoba into southwest Ontario. Northeast of the flexure axis, the landscape slopes northeast to Hudson Bay and southwest of the axis the landscape slopes southwest to the pre-glacial Pliocene Hatfield-Spiritwood River and Mississippi River. We interpret the flexure axis to be the northern divide of the pre-glacial Pliocene Mississippi River in Manitoba and western Ontario.