Thursday, November 16, 2006 - 8:20 AM

Model for Historical Rates of Stream Channel Down-Cutting in the Blue Earth River, Minnesota.

David Mulla and David Bauer. Dept of Soil, Water & Climate, Dep. of Soil Water & Climate, 1991 Upper Buford Univ. of MN, St. Paul, MN 55108

An exponential decay model was developed to estimate rates of stream channel down-cutting in the Blue Earth River of Minnesota as a function of time since the last glaciation.  The last glacier retreated from Minnesota 13,000 years ago, leaving the Des Moines lobe consisting of glacial till sediment.  During retreat, stagnating ice blocked the drainage of the young Blue Earth River, leaving a vast region of flat lacustrine sediments.  Soon after, huge volumes of water released in a catastrophic event from Glacial Lake Agassiz cut a deep, wide channel in the Minnesota River at the mouth of the Blue Earth River.  Ever since this time, the Blue Earth River has been down-cutting to reach the elevation of the Minnesota River.  Digital elevation models were constructed using GIS of the present day elevations in and around the channel of the Blue Earth River in southern Minnesota.  After glaciation this topography was flat, so differences between present day and historic elevations are the volume of sediment removed by historical rates of down-cutting.  Using measurements of present day stream bluff down-cutting (106, 213 tons/yr), soil bulk density and mathematical iteration it was found that historical rates of stream channel down-cutting (Mt) at any time t could be described using the model: Mt = Mo * exp (-kt), where Mo (573,840 tons/yr) is the initial rate of down-cutting 13,000 years ago and k (0.0013) is a decay coefficient.  This model shows that the rate of down-cutting has decayed exponentially since the end of glaciation and historic rates of down-cutting were roughly 5 times greater than present day rates.