DETERMINING THE MOORHEAD LOW ELEVATION OF GLACIAL LAKE AGASSIZ USING COMPACTION RIDGES

The rate of the regression of glacial Lake Agassiz and its lowest elevation during the Moorhead Phase are poorly constrained. Strandline and compaction ridge locations in the Red River Basin were used to determine the lowstand position. Compaction ridges are low relief ridges that trace former river channels. Compaction ridges form as a result of the differential dewatering between the Poplar River Formation and its upper and lower boundary the Sherack and Brenna Formations. 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. In addition to mapping compaction ridges, unfilled Moorhead Phase channels in the southern region of the basin were also mapped. Many of these channels are linear with many not occupied by modern streams. 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 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. With LiDAR data, more compaction ridges were mapped then in previous studies using topographic maps and aerial photography. Compaction ridge locations in the areas around Grand Forks and Fargo, ND were often oriented congruent with modern hydrology buteasterly shifted. The compaction ridges of the former Red River begin to taper off and disappear around 255 meters asl. The termination of compaction ridges at 255m asl. shows a correlation with the emergence of iceberg scours at similar elevations. This result could indicate: 1) a reduction of flow rates leading to smaller channels thus smaller compaction ridges; 2) destruction of ridges below 255m as lake level rose from the Moorhead low, 3) lake level did not fall below 255m, or 4) destruction of compaction ridges due to iceberg activity. Termination of compaction ridges at 255m provides a maximum elevation reached by Lake Agassiz during the Moorhead Low. Initial results suggest that Lake Agassiz during the Moorhead Phase did not recede beyond the 49^th Parallel.