ASSESSING HISTORICAL PLANFORM CHANNEL CHANGE WITHIN AN ALTERED WATERSHED: MINNESOTA RIVER, MINNESOTA, USA.

The Minnesota River of south-central Minnesota flows ~540 km through an agricultural watershed beginning at Big Stone Lake in west-central Minnesota and ending at the confluence with the Mississippi River near St. Paul, MN. Over the past 150 years much (78%) of the Minnesota River basin (MRB) has been converted from native vegetation, like wetlands and tall-grass prairie, to agricultural land/row crops. Land cover change and modified subsurface drainage has altered MRB hydrology. Also, climate change is resulting in greater annual precipitation throughout the state. As a result, the MRB has observed an increase in peak flows due to summer rain events, the number of days with high flows, and increases in low/base flows. In addition, sediment cores from Lake Pepin on the Mississippi River show an order of magnitude increase in sedimentation rates over ~150 years – with >80% contribution from the MRB. As a result of increased sediment loads derived from more erosive rivers and nutrient loading from agricultural land use practices, considerable research has been conducted to help understand a number of associated environmental issues in and downstream of the MRB. Despite this growing body of work, very little is known about the impact anthropogenic activity has had on the morphology and behavior of the Minnesota River through time. The primary focus of this research is to quantify historical planform channel change (e.g. lateral migration rates, channel width, channel length, channel geometry) on the Minnesota River using aerial photographs dating to 1937 through an array of geospatial techniques utilized in ArcGIS. A secondary focus of the research is to help address some of the methodological incongruities present throughout the literature that utilize remote, geospatial techniques in planform analysis. By linking the context provided in former studies and understanding the uncertainties present, the aim is to provide accurate, quantified data to better understand the processes controlling the Minnesota River’s planform morphology through time. Initial results reveal a significant decrease in channel length, increase in channel width and spatial variability consistent through time in lateral migration rates and sinuosity through the study reach.