USING LIDAR DATA TO RECONSTRUCT THE HISTORY OF GLACIAL LAKE AGASSIZ

This study evaluates the lake level and outlet history of glacial Lake Agassiz as recorded by 5500+ km of strandlines digitized from lidar digital elevation models from North Dakota and Minnesota. The highest and oldest strandlines are associated with a period of gradually falling lake levels known as the Lockhart Phase (i.e. the Herman, Norcross, and Tintah levels.) Lake levels fell due to sill incision at the southern outlet and glacial isostatic adjustment. Some of these lake levels project to potential eastern outlets into the Lake Superior basin (where lidar data is not available), but if eastern diversions of Lake Agassiz occurred during the Lockhart Phase, they were short-lived and did not initiate dramatic drops in lake level. The Lockhart phase ended with the Moorhead Phase when a much lower outlet became ice free, causing lake levels to drop by 50+ m over a short period of time. A lower outlet and rapid lowering is inferred by a vertical gap in the strandlines between which no evidence exists for intervening lake levels. The outlet responsible for the drawdown is most likely the northwest outlet to the Arctic Ocean, because potential eastern outlets into the Lake Superior basin do not appear to provide the vertical separation necessary to create such a large gap. However, the physical evidence from the northwest outlet, which includes radiocarbon dated material from within the spillway, does not support this interpretation. During the Moorhead Phase, lake levels should have gradually risen south of the northern outlet due to glacial isostatic adjustment. Rising lake levels culminated in the Campbell level when water levels reached the southern outlet (beginning the Emerson phase.) The Emerson phase ended when lower eastern outlets into the Lake Nipigon and Lake Superior basins became ice free. Continued ice margin retreat from the region west of the Lake Nipigon basin resulted in a succession of lake level drawdown events that can be inferred from a series of previously identified strandlines with clear vertical gaps (e.g. McCauleyville, Blanchard, Emerado, Ojata, and Gladstone.) Unlike the older Lockhart Phase lake levels, which have no vertical gaps, these Nipigon Phase strandlines are easy to distinguish and correlate across the mapping area.