ASSESSING THE LOCATION OF GLACIAL LAKE AGASSIZ DURING THE MOORHEAD PHASE WITH COMPACTION RIDGES

The rate of the regression of glacial Lake Agassiz and its lowest elevation during the Moorhead low-water Phase are poorly constrained. Strandline and compaction ridge locations in the Red River Basin are used to assess the lowstand position. Compaction ridges are low-relief ridges that develop from differential compaction of mud that encases a core of fluvial sand and gravel and records the position of streams that developed across the lake bottom during the lowstand. LiDAR data gathered from the Red River Basin Mapping Initiative was used in conjunction with ESRI’s ArcGIS ArcMap 10.2 to construct DEMs of the Red River Basin south of the 49^th parallel. Compaction ridges were identified by observing their basic geometry—a sinuous ridge normal to the orientation of identified shorelines. Shading methods were used to test for, and identify strandlines and compaction ridges based on their inherent unique geometry. Relative age of compaction ridges to strandlines were determined by the principle of crosscutting relationships. Compaction ridge locations particularly in the areas south of Grand Forks, ND and around Fargo, ND were often parallel with the modern hydrology but shifted to the east. The compaction ridges of the former Red River and its tributaries begin to taper off and disappear around 255 meters above sea level. This result could indicate: 1) a reduction of river discharge leading to smaller channels, and thus smaller compaction ridges were not preserved; 2) destruction of ridges below 255 m from ice berg scours, and wave action as lake level rose from a lowstand below 255 m; or, 3) lake level did not fall below 255m. Preliminary results from mapping compaction ridges suggest that Lake Agassiz during the Moorhead low-water Phase did not recede north of the 49^th parallel.