HOLOCENE DRAINAGE EVOLUTION OF THE MISSISSIPPI HEADWATERS, MINNESOTA: IMPLICATIONS FOR MID-HOLOCENE EOLIAN ACTIVITY IN THE NORTH AMERICAN MIDCONTINENT

An ~580 km² dunefield located southeast of Lake Winnibigoshish (LW) in the Mississippi Headwaters (MH) watershed of Minnesota has previously been dated between 7.9 and 5.0 ¹⁴C kyr bp (Grigal and others, 1976). Its occurrence and age are widely cited in support of widespread eolian activity during the mid-Holocene in the north-central US. However, recent work indicates that only a relatively small portion of the dunefield (~8 km²) was active during the mid-Holocene, arguing for local-scale forcing of the initiation of eolian activity.

Raised shorelines and abandoned stream channels indicate that the early to mid-Holocene MH drainage network was significantly different from the modern one. The initial post-glacial MH drainage pattern was characterized by north-south tributaries feeding the Mississippi River and bypassing LW. Later, a succession of stream piracy events diverted the Mississippi through LW. A horizon associated with diversion of the Mississippi into LW has been dated at 1.0 ¹⁴C kyr bp. The other stream piracy events likely occurred around the same time because of positive feedback between lowering lake level and initiation of subsequent events.

During the early and mid-Holocene, LW was characterized by a catchment:lake area ratio of ~5:1, significantly lower than any other lake in the MH region. Warmer, drier climate during the mid-Holocene resulted in higher rates of evaporation from the free water surface of the lake, while increase in terrestrial evaporation was ameliorated by a transition to more xeric vegetation. LW’s low catchment:lake ratio resulted in hydrologic closure and lake level instability even while levels of the other major lakes in the watershed remained stable. Eolian activity was triggered by periodic lake level drop, wind erosion of exposed littoral sand, and subsequent transport onshore to form dunes.

The formation of dunes adjacent to LW during the mid-Holocene was largely a function of the lake’s anomalous hydrologic setting at that time. Their existence is evidence neither in support of nor against widespread eolian activity and dune formation during the mid-Holocene, either elsewhere in the MH region or more generally in the north-central US.