A HISTOGRAM ANALYSIS OF STRANDLINES AT THE SOUTHERN OUTLET OF GLACIAL LAKE AGASSIZ

Four beaches are commonly described from the southern basin of glacial Lake Agassiz: Herman–1060’, Norcross–1040’, Tintah–1020’ and Campbell–980’. However, other strandlines above and between these beaches complicate efforts to trace water planes northward. Some intervening strandlines have been described as offshore bars, or otherwise viewed as representing fluctuating lake levels. Geomorphic data (3 deltas, 24 escarpments, 25 spits, and 568 beach ridges) were derived from mapping twenty-two 7.5’ topographic quadrangles with 5’ contour intervals. Strandlines are not distributed evenly across the lake basin. For example, the La Mars quadrangle contains 56%, 21% and 24% of the spits, scarps and ridges, respectively. Moreover, the lake basin gradient fluctuates across the basin and more strandlines are found on steeper slopes. A histogram of aggregate strandline length vs. elevation is used to further the understanding of lake levels and reveals a multimodal distribution. The two strongest modes center on the Herman and Norcross levels at 1065’ and 1040’, but with the Herman mode skewed to higher elevations. There is a minor peak of beach ridges and a delta at 1020’ corresponding to the Tintah level, but the closest spit is at 1030’. The next mode in beach ridges is at 1005’ in association with two small deltas and an escarpment. This mode is more strongly peaked than the Tintah, suggesting that there was another relatively stable lake level at 1005’. The Campbell strandlines are underrepresented in this analysis because of the small area with Campbell elevations in the study area. The frequency analysis indicates that: 1) water depth and basin gradient influenced strandline development, 2) the classic Herman and Norcross levels were relatively stable compared to the classic Tintah level, which is weakly expressed geomorphically, 3) there is a previously unreported, relatively stable water plane at 1005’ and, 4) strandline development within the Agassiz basin becomes less obvious below the Norcross level. This is likely a result of an increased lake area with greater fetch distances, and fluctuations in climate and meltwater delivery. It appears that histogram analysis is an effective method to establish relatively stable water planes among large numbers of strandlines.