Over 15,000 years ago, the enormous weight of the ice lobes of the Wisconsinan glacier changed the normal drainage pattern of rivers flowing to the Great Lakes, and tilted the shores of the glacial lakes upwards to the northeast. Lowlands were uncovered, and the present Great Lakes drainage system was formed. Deposits of glacial till became the moraines, eskers, and drumlin fields that characterize and influence the natural history of the Grand River watershed.
During long periods of thawing, meltwater flowed into many successions of glacial lakes in extensive low-lying areas near Lake Erie. Sand and clay particles carried by meltwaters settled on the ancient lakebeds to become the clay plains of the lower Grand River valley.
Ontario Island, a high flat area of hard bedrock south of the present Georgian Bay, was the first area of land to appear after the last glacier. This included the Dundalk Uplands Plain, source of the headwaters of the Grand, Saugeen and Maitland Rivers. From here, the land sloped south to Lake Erie, and west to Lake Huron, with the long spine of the Niagara Escarpment forming a natural drainage barrier to the east.
A network of drainage channels formed where the hard bedrock of the Dundalk Uplands Plain was scored by repeated movements of ice. Because of the shallow depth of glacial till and the low porosity of the bedrock, meltwater or rain ran from the plain in these channels, or collected in poorly drained depressions. Many of these depressions became the bogs and swamps of Luther and Melancthon Townships.
The Grand River flowed south, through the physiographic regions of the Dundalk and Stratford Till Plains, carving a valley through the bedrock of the upper watershed, and the gravel terraces left by meltwater spillways. In the Elora Gorge area, the river cut deep into the dolostone, producing canyon-like walls and rugged beauty.
In the central part of the basin, the Grand River created a wide, winding valley through gravel glacial deposits. Here, the Grand River is joined by the Conestogo River, a major tributary draining the upper west side of the watershed.
Much of the central part of the watershed is a region of moraines, drumlins and sandy hills. These include the Waterloo Sand Hills, the Guelph Drumlin field, the Waterloo Moraine, and parts of the Galt and Paris Moraines. In the moraines, rainwater and snowmelt, seeping down through porous higher ground, is contained by an impervious layer of clay over bedrock to form aquifers. Groundwater reservoirs found in the central Grand River valley include the large Waterloo-Mannheim aquifer. Other post-glacial features of the central region include the Baden kames, the Rockwood potholes, and the Wrigley-Bannister Lake complex.
The Speed and Eramosa Rivers drain the upper eastern lands of the watershed and join the Grand River in the middle basin. Further south, the Nith River flows from the central western side of the watershed to join the Grand River as it moves over the delta of the former Lake Warren. The soft silts and clay of the Norfolk Sand Plain allow the river to meander freely, eroding and depositing bank soil with its movement. To the south, a mixture of settled till and clay layers in the old lake beds created the wide, flat, poorly-drained area of the Haldimand Clay Plain.
The journey of the Grand River ends at Lake Erie, where it becomes part of the much larger watershed of the Great Lakes, and one of many rivers that enter and replenish the largest freshwater system in the world.
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