The Spring Runoff Event, Thermal Bar Formation, and Cross Margin Transport in Lake Superior

Abstract

Retention of materials delivered during the spring runoff event in the nearshore is mediated by physical phenomena such as the thermal bar. Here we investigate the timing of the runoff event vis-à-vis formation of the thermal bar and quantify its impact on the fate of materials of terrestrial origin (TSS, total suspended solids) in the nearshore waters of Lake Superior. It was estimated that the spring runoff event delivers an average of 70% of the annual TSS load for a 33-year period of record (1966–1998). The average dates for the midpoint of the spring runoff event and thermal bar formation are 9 April (range, mid-March to late April) and 2 May (range, early April to late May), respectively. This offset of 3–4 weeks provides an opportunity for materials which are discharged and retained in suspension by wave action to be transported from the nearshore before trapping by the thermal bar can occur. Model calculations indicate that 23 ± 28% (range < 1% to > 99%) of the spring runoff event TSS loading remains available for trapping at the time of thermal bar formation. In terms of mass, predicted retention varied from negligible amounts to in excess of 700,000 metric tons. The spring runoff event is predicted to have the potential to raise average nearshore TSS levels more than 4.1 ± 2.7 times above background levels and to sustain that elevation for more than 60 days. The range in the nearshore TSS response reflects interannual variability in the magnitude of the spring runoff event and its timing relative to the formation of the thermal bar.