CHRONOLOGY AND EVOLUTION OF THE MIDDLE AND LATE HOLOCENE LAKES IN THE UPPER GREAT LAKES: UPLIFT AND OUTLET INCISION

The Holocene in the upper Great Lakes is marked by lake-level changes related to N-S differential isostatic rebound and outlet erosion. Early Holocene lakes drained through an isostatically depressed northern outlet at North Bay, Ontario, that by 5 kyBP rose above and shifted drainage to the present southern outlet at Port Huron, Michigan. Thus, the basin was first characterized by a regional transgression that culminated just after 5 kyBP with Lake Nipissing (+6.5 m from modern). This was followed by a regional regression to modern Lakes Huron-Michigan and Superior. The regression from Nipissing to modern lakes, completed by 2.5 kyBP, is generally believed to have occurred over a 2000-year interval as the drift-floored Port Huron outlet eroded incrementally from Nipissing through Algoma (+3.5 m) to modern levels.

Data from archaeological sites near Saginaw Bay in southern Lake Huron show that the peak Nipissing level occurred after 4.6 kyBP and was probably below the oft-cited 184.4 m altitude. Furthermore, the post-Nipissing regression was rapid and Lakes Huron-Michigan formed just after 4 kyBP, ca. 1500 years earlier than usually suggested. Once formed, the modern lakes maintained a stable, lower-than-present level until about 2 kyBP, after which cyclical, millennial-scale, climatically controlled, 1-2 m variation in lake level began.

The Holocene outlet configuration in the upper Great Lakes resulted in important N-S differences in shoreline conditions. For example, while early Holocene lake levels appeared relatively stable in the northeast part of the basin because local rebound was similar to that at North Bay, lake level in the southern part appeared to rise at the rate of North Bay rebound. The post-Nipissing regression, controlled by outlet erosion at Port Huron (ca. 60 cm/century), was greatest in the north where rebound rates were highest and the apparent regression (local differential rebound plus Port Huron erosion) was up to 1 m/century. Outlet erosion ended after 4 kyBP, and while beaches in the south stabilized, uplift-controlled regression actually continued in the north. The rapid erosion followed by channel stability since 4 kyBP suggest that a resistant drift layer must floor the Port Huron outlet. Regular contemporary channel dredging may change this equilibrium and reinitiate channel erosion.