Lake of the Woods Shoreline Erosion: Analysis of Historical Shorelines, Climate and Lake Level

Abstract

This report summarizes the results of Phase I of a study of erosion of the Minnesota shoreline of Lake of the Woods. The overall objectives of this study are (a) to determine the causes and to estimate the magnitude of the shoreline recession rates in the US shorelines of Lake of the Woods, and (b) to recommend management practices for shoreline protection against erosion. In Phase I, we collected historical data on wind and water levels at Lake of the Woods, flow and suspended sediment input from the Rainy River, and information on the shoreline, including aerial photos, satellite images, and soil surveys. Analyses of aerial photos from 1940 to 2003 show rapid erosion of several undeveloped wetland areas of the shoreline and relatively slow erosion of developed areas along Sandy Shores and Birch Beach. Analysis of Pine and Sable Islands show a combination of erosion, rebuilding, and shifting from 1940 to 2003, so that the present state of the islands may represent either a long term loss or a loss/rebuilding cycle. Since long term wind records for Lake of the Woods were not found, a synthetic wind record was constructed from regional wind records. Analysis of wind and water level data from the 1950’s to the present show a relatively uniform distribution of high wind and high water events. Recent high water events appear as typical events that take place several times per decade. Wind and wave data were also collected at two locations on the southern side of Big Traverse Bay, with record lengths of 4 – 5 weeks. The on-lake wind data and wave data will be useful to calibrate wave models in Phase II, and have been used in Phase I to correlate on-lake wind with local and regional off-lake wind measurements. Field measurements of near-shore bathymetry were made and sediment samples were collected to determine size distribution were also collected in preparation for Phase II. Preliminary analyses of data for the Rainy and the Little Fork rivers do not show distinct trends in flow rate or suspended sediment concentration for the period of record.