THE MOORHEAD LOW: THE FIRST PRAIRIE DROUGHT?

One problematic time in the history of glacial Lake Agassiz is the so-called Moorhead Low phase. This interval, recognized for over 100 yr. as being below the southern outlet elevation, implies changes in the hydrology of the lake. The most common hypothesis for it is catastrophic lowering following a change in drainage pathways. However, with recent dating of lake levels and organic material that defines the Moorhead Low, the temporal constraints for a sudden water lowering are problematic. Thus an alternative explanation is needed.

Toward that end we have reconstructed the geometry of glacial Lake Agassiz at the so-called Tintah (age of 12.9-13.4 ka) and Burnside (age ~12.0) levels. Despite a lower water level the surface area of the lake increased by a factor of 4.3 and the volume of the lake increased from 2834 to 12221 km³. Thus any explanation of the low phase should consider a lake size increase over a ~1000 yr period.

Give that many lakes in the prairies have dry episodes because of high regional E/P values, that Lake Huron and other Great lakes were closed basins in the Holocene, and that these lakes have very high surface area/volume values, we hypothesize that net evaporation could have lowered Lake Agassiz to the Burnside level instead of opening a lower outlet.

The primary objection to this possibility is that the meltwater production would greatly exceed evaporation rates. However, this lowering occurred about the same time as the Younger Dryas, which in many places is recorded as a dry, cold interval with increased solar radiation. The dry air would reduce rainfall and enhance evaporation, the cold would reduce the meltwater production, and the shortwave radiation would enhance evaporation when the lake was not frozen and sublimation when the lake was ice covered. We suspect that a modeling test supporting this hypothesis may hinge on the selection of air humidity values, yearly duration of lake ice cover, annual temperature values, and geometry of any meltwater delivery from the ice sheet, and that modern values may not be applicable.