Geology and Topography

Geological History of the Huron River Watershed

In the Paleozoic Era of geologic time, the North American continent was inundated by ancient seas.  Over several periods spanning 570 to 340 million years ago, these marine environments deposited a variety of sedimentary bedrock – sandstones, shales, carbonate-rich rocks, rock salt, and gypsum – in the Michigan basin.  At the onset of the Great Ice Age (approximately two million years ago), continental glaciers from the Hudson Bay area advanced into the Michigan region, and the existing bedrock guided two major lobes of glacial ice that became part of the Wisconsin Glacier.  As the glacial climate began to wane, and the rate of melting increased, the glacier receded and the thinner ice zone between the lobes began to waste away.  Around 14,000 years ago, the earth below this zone was exposed, meltwater streams began to flow, and the first portion of what is now the Huron River Watershed emerged.

As the glacier’s enormous mass advanced, it scraped, shoved, and heaved the underlying earth, churning up particles as small as sand and as large as boulders.  This forward  movement created distinctive land forms known as terminal or end moraines at the “toe” of the glacier.  When the climate warmed, the ice mass melted and receded, leaving behind the more subtle land forms we call ground moraines.

In the path of their retreat, the glaciers left a rich and varied debris of sand, gravel, and clay known as glacial till.  The end moraines, in particular, are areas where glacial processes deposited very large quantities of rock and soil material of various sizes in one place.  This mix of particle sizes increases the soil’s ability to hold moisture and nutrients, and is conducive to agriculture.  We find coarse-textured end moraines (with moderate permeability) primarily in the northern and western parts of the watershed, and medium-textured end moraines (with low permeability) around the watershed’s periphery.

The melting glaciers also created glacial outwash plains where the meltwater runoff sorted soils into layers of similarly sized particles, including sand and gravel – particles that facilitate the rapid infiltration of surface water to groundwater aquifers and stream systems.  Both glacial outwash plains and the coarse- to medium-textured end moraines described above characterize much of the Huron River Watershed.

The earliest ancestor of the Huron River was a glacial stream that appeared in the vicinity of modern southeastern Livingston County and flowed westward and southward ultimately to join the Illinois and Mississippi River systems.  Over the next four thousand years, the early Huron River changed course several times, and reversed course at least once, responding to the stops and starts of the retreating glaciers, the topographical demands of the end moraines, and the meltwater’s need to find the lowest elevation.

Between 14,000 and 13,500 years ago, the Huron River’s final shift in course established its direct drainage southeast to old Lake Maumee, just north of today’s Tecumseh.  This event marked the close of the first phase of the river’s geologic history, when ice margins and land forms dictated most of its development.  During the subsequent phase, the evolution of the Huron River and its watershed was defined largely by the fluctuating levels of the various lakes in the Erie basin.  For most of the past 10,000 years, the Huron River and its watershed have remained (more or less) in geologic equilibrium.

Current Topography

In Michigan’s Lower Peninsula, glacial movements determined the modern topography of the Huron River Watershed, defining the upper and lower basins that we recognize today.  The upper basin (over 730 square miles) stretches from the Huron River’s headwaters at Big Lake in Oakland County southeast to the Ann Arbor-Ypsilanti corridor.  This basin is characterized by rolling hills and a well-defined pattern of tributaries, lakes, and wetlands.  Beginning at Ypsilanti, the lower basin (approximately 150 square miles) is narrow and flat, resting as it does in the ancestral bed of Lake Erie.  This sharply narrowing lower basin contains an ever-widening Huron River, one that moves more slowly toward its point of entry into the great Lake Erie.

Dave Wilson
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