The Great Lakes Region

Although it is common to speak of five Great lakes (Superior, Michigan, Huron, Erie, and Ontario) several more are part of the same group based on physiographic setting and origins. Among these are Great

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Kilometers Data from the North American Atlas

Figure 1 Major lakes and reservoirs of North America.

Bear Lake, Lake Winnipeg, Lake Athabaska, Great Slave Lake, and Reindeer Lake. These ten total about 340 000 km2 in area, and all are among the 25 largest lakes in the world (Table 1). They lie at or near the southern and western edge of the Canadian Shield, where it is overlapped by sedimentary rocks of the interior plateaus and Great Plains. Their location also coincides with the distal portions of the Laurentide ice sheets that formed repeatedly during the Pleistocene, spreading from the general region of Hudson's Bay. Most were formed by a combination of glacial scour and damming by terminal moraines.

Large as these lakes are, most were much larger in the past. Because the Canadian Shield lay under a great thickness of glacial ice during much of the Pleistocene, it was isostatically depressed below the elevation of the surrounding unglaciated areas. As the ice melted, huge lakes formed at the edge of the melting ice mass. Some of these overflowed to the mid-continent, draining either to the ancestral McKenzie

River or to the Mississippi, while others followed ice-marginal paths to the Arctic or the Atlantic via the Gulf of St. Lawrence. The Shield region rose with isostatic rebound, tilting these lake basins to the south and west. This tilting, combined with downward erosion of outflow channels gradually drained the lakes, reducing them to their current extent. Thus, many of the Great Lakes are bordered by large areas of former lake bed, exposing lake-bottom sediments and former shorelines.

The system of Great Lakes along the US-Canada border occupy a very high proportion of the watersheds within which they lie. Superior's land drainage area is only 1.55 times the area of the lake itself, and for the entire system from Lake Ontario upstream the ratio of land area to lake area is 2.13. They thus dominate the hydrology of the St. Lawrence River watershed. Superior is the highest (elevation 184 m above sea level) and largest, with an average depth of 147 m and a volume that exceeds that of the other

Figure 2 Lake regions of North America.
Table 1 Large Lakes of North America

Name

Area

Volume

Rankin world

(km2)

(km3)

(by area)

Superior

82100

12100

2

Huron

59600

3540

4

Michigan

57 800

4920

5

Great Bear

31 000

2292

8

Great

27 000

2088

10

Slave

Erie

25 700

484

11

Winnipeg

24500

371

12

Ontario

18 960

1640

13

Nicaragua

8150

108

23

Athabasca

7935

110

24

Reindeer

5597

5

25

four combined. It drains into Lake Huron via the St. Mary's River. In 1921, a dam was built to regulate the lake's level and generate electricity, and locks were built at Sault Ste. Marie to facilitate shipping.

Huron and Michigan are actually one lake with a common surface elevation of 177 m, separated into two basins by the Mackinac Strait. Huron averages 59 m deep and is the larger of the two in area, while Michigan has a greater volume with an average depth of 85 m. They drain into the St. Clair River, which flows to Lake St. Clair and feeds the Detroit River, which flows to Lake Erie. The total elevation drop between Huron and Erie is 3 m, and flow through the St. Clair-Detroit River system is naturally regulated. Lake Erie (elevation 174 m) drains into Lake Ontario (elevation 75 m) via the Niagara River, with most of the elevation change accounted for by Niagara Falls. The level of Lake Ontario is regulated by an outlet structure on the St. Lawrence River. Erie and Ontario are each less than half the area of Huron or Michigan. Erie is considerably shallower (averaging 19 m) while Ontario has an average depth of 86 m. The total volume of these five lakes is roughly 20% of the total volume of freshwater lakes in the world.

Because of their large surface areas relative to drainage area the lakes exhibit long lags times in their response to variations in climate. Water levels vary up to 2 m at the decadal-scale, driven by interan-nual variations in precipitation and evaporation. In recent decades the lowest lake levels occurred in the mid-1960s, while high levels characterized the 1950s, mid-1970s, and mid-1980s. These lake level variations cause a variety of problems for shipping and shoreline management, with high levels being preferred for navigation purposes but lower levels desired by coastal residents because of reduced coastal erosion. The ability to regulate levels in Lakes Superior and Ontario has very limited effects on lake levels in the other lakes.

At the seasonal scale, Great Lake levels vary up to 0.5 m, with the lowest lake levels occurring in late summer or autumn and high levels in late spring or early summer. Evaporation losses are greatest in the autumn, when water temperatures are warm and yet the temperature of the overlying air is low. This temperature differential between the lake surface and air also contributes to the creation of highly localized zones of heavy 'lake-effect' snowfall on the southeast and east shores of the lakes. This effect is greatest in early winter, and smaller in late winter when lake surface temperatures are lower.

In addition to the several very large lakes of the Great Lakes Region are many thousands, and possibly millions, of smaller lakes. Some of these are simply depressions in an undulating surface of ground moraine; others called kettle lakes were formed by burial and later melting of large blocks of glacial ice, while still others such as the Finger Lakes of New York were formed by tongues of ice that scoured preexisting valleys. The northern portions of Minnesota, Wisconsin, and Michigan have tens of thousands of lakes, many of which are kettle lakes. One of the highest concentrations of these is found in Vilas County, Wisconsin, which has over 400 named lakes in an area of 2260 km2. Southern New England is another area with a high concentration of lakes in a glacial depositional setting.

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