Pliocene Drainage in Manitoba and Northwestern Ontario, Canada

This study presents the restored Pliocene topography along northerly trending profiles in Manitoba, northwestern Ontario, eastern North Dakota, Minnesota, and western Wisconsin. A top-of-bedrock slope profile of the Red River/Lake Winnipeg/Nelson River and 19 south-trending landscape topographic and top-of-bedrock slope profiles of the region were made from published elevation data. These profiles reveal a clear change in slope and slope direction (flexure) in the top-of-bedrock elevation. The flexure axis trends southeasterly across Manitoba into northwestern Ontario. The profiles were then tilted southerly by lifting the profiles along the southern margin of Hudson Bay to restore remaining glacial isostatic rebound. When imposing a minimum 80 m of isostatic rebound to the Nelson River/Lake Winnipeg/Red River profile, the northern portion of the profile slopes north but the southern portion of the profile slopes south at 0.01%. When imposing a minimum 60 m of isostatic rebound to all 19 top-of-bedrock profiles, the bedrock slopes north to Hudson Bay northeast of the flexure; however, southwest of the flexure, the southern portions of the 19 top-of-bedrock profiles slope south. We interpret the flexure axis to have been the northern divide of the Pliocene Mississippi River basin in Manitoba and northwestern Ontario and the divide that will lie between the north-flowing Nelson River and south-flowing Red River with future glacial isostatic adjustment.