P1: GIM TJ1095-03 TJ-AEM.cls May 27, 2004 9:29 Morphometric factors in the formation of Great Lakes coastal wetlands C. E. Herdendorf Department of Geological Sciences, The Ohio State University, Columbus, Ohio 43210, USA; E-mail: [email protected] The Great Lakes basins were carved from ancient river valleys by continental ice sheets that receded from the region less than 10,000 years ago. Not only did the glaciers create the basins now holding the lakes, but they are responsible for many of the shallow depressions in the coastal margin that have since developed as coastal wetlands of various types. For the past four thousand years, coastal processes in the lakes have further modified the shore topography to form embayments, coastal lagoons, estuaries, deltas, and solution basins where thousands of hectares of wetlands have become established. This paper will explore the origin of the various morphometric forms which these wetlands have taken and their characteristic hydrologic processes. Keywords: estuaries, geomorphology, karst, lacustrine, palustrine, physiography Physiography of the Great Lakes gin of the waning ice sheet retreated northward into the newly carved lake basins, some of which were dammed The five adjoining Laurentian Great Lakes— by glacial end moraines. The early ice-margin lakes ex- Superior, Michigan, Huron, Erie, and Ontario—extend panded as the glacial ice masses shrank. However, as 1,370 km from westernmost point to easternmost point new and lower outlets were uncovered to the north, the and 1,130 km from north to south (Figure 1). With lakes drained to ever lowering levels except during peri- a total surface area of 244,160 km2, this is the largest ods of minor readvances of the ice front (Hough, 1962). freshwater system on earth. The total shoreline of these Following deglaciation, the Earth’s crust rebounded lakes measures 17,017 km and is nearly equally divided from the hundreds of meters of depression experienced between Canada and the United States, although Lake under the weight of the ice masses, causing old outlets Michigan is totally within the United States and Lake to be closed and the levels of the lakes to again rise, Huron’s Georgian Bay lies completely in Canada. The but not as high as their initial levels. Continued uplift lakes in this immense system contain about 22,700 km3 of the land to the north and erosion of the shores and of water or nearly one-fifth of the all the freshwater on outlet channels have continued to cause slow changes the planet. Lake Superior contains 53% of that water; to the configuration of the lakes. Lake Michigan, 22%; Lake Huron, 16%; Lake Erie, The watershed of the Great Lakes is about 764,000 2%; and Lake Ontario, 7%. km2 and extends over parts of three different physio- The Great Lakes are located in zones of weaker sed- graphic (landform) provinces—Canadian Shield, Cen- imentary rocks that were excavated for many millennia tral Lowlands, and St. Lawrence Lowlands. The by steam erosion. Major valleys were deepened and streams that flow into the lakes reflect the character and reshaped by glacial ice during the Pleistocene Epoch. variations of these regions. The rivers that enter Lake The lakes originated late in this epoch when the mar- Superior and the northern parts of Lakes Michigan and 179 Aquatic Ecosystem Health & Management, 7(2):179–197, 2004. Copyright C 2004 AEHMS. ISSN: 1463-4988 print / 1539-4077 online DOI: 10.1080/14634980490461515 P1: GIM TJ1095-03 TJ-AEM.cls May 27, 2004 9:29 180 Herdendorf / Aquatic Ecosystem Health and Management 7 (2004) 179–197 Figure 1. St. Lawrence Great Lakes and drainage basin (NOAA, Great Lakes Environmental Research Laboratory). Huron change elevation rapidly as they descend from south ridges of bedrock flanked by sediment-filled val- the rocky cliffs of the Canadian Shield to the lakes in leys, thought to be the remnants of a preglacial drainage many falls and rapids. Rivers flowing into the south- system. The Keweenaw Peninsula, Apostle Islands, Isle ern Lakes—Michigan, Erie, and Ontario—originate in Royale, and connecting submarine ridges are outcrops the Central Lowlands and are more likely to flow gently of ancient Precambrian volcanic and sedimentary rocks through well-defined channels and to have broad flood- which are more resistant to erosion. The southern bor- plains. The St. Lawrence Lowlands is restricted to the der of the eastern basin consists of Paleozoic strata, wide, flat valley of the river for which it is named. most noteworthy being the colorful sandstone cliffs of Pictured Rocks National Lakeshore. Lake Superior lies Lake Superior almost wholly within the Precambrian Canadian Shield, the Paleozoic Central Lowlands rocks of the southeast- With a volume of 12,230 km3 and a surface area of ern shore being the only exception. 82,100 km2, Lake Superior is the largest of the Great An escarpment borders the shore of Lake Superior Lakes. The lake bottom is divided into two basins; the which rises 120 to 240 m above the surface of the lake Keweenaw Peninsula and a prominent north-south sub- on all sides. Interspersed along this precipitous coast of merged ridge at depths of 150 to 180 m separate the spectacular cliffs are small, rocky, pocket beaches. Off- eastern and western basins. The western basin is char- shore the lake bottom rapidly drops to depths greater acterized by a comparatively smooth bottom consist- than 60 m. In the vicinity of Whitefish Point the bot- ing of thick lake sediments and glacial deposits, while tom slope is more gentle and the shore is characterized the eastern basin is more rugged with numerous north- by shallow reaches composed of sand, derived from P1: GIM TJ1095-03 TJ-AEM.cls May 27, 2004 9:29 Herdendorf / Aquatic Ecosystem Health and Management 7 (2004) 179–197 181 nearby glacial deposits, that have been transported into a depth of 60–90 m at mid lake, a deep north central the area by alongshore currents (Upchurch, 1976). The basin drops to the lake’s maximum depth of 281 m. maximum depth of the lake, 406 m, is located east of The bottom of this basin has an irregular floor caused Keweenaw Peninsula. by outcrops of resistant Devonian limestones separated Lake Superior owes its origin to a combination of by Devonian declivities (Upchurch, 1976). A northeast- events. First being the formation of a midcontinent ern basin consists of numerous north-south trending rift and associated igneous activity in the Precambrian valleys and ridges similar to those of eastern Lake Su- Era. This rift was later filled with soft sedimentary perior. This basin contains a number of islands and its rocks which eventually became eroded into a ridge bottom is characterized by a number of deep troughs, and valley system. The older, harder rocks around the 75 to 150 m deep, separated by ridges with only 8 to edges of the Superior basin (lava flows, gabbros, and 15 m of water over them (Hough, 1958). Green Bay, granites) form parts of the bordering escarpment and with a surface area of 4,100 km2, constitutes a fourth Keweenaw Peninsula. Faulting may have also weak- physiographic element of Lake Michigan. Green Bay is ened the bedrock and formed graben-type fault-block a relatively shallow embayment, mostly less than 30 m depressions. Lastly,as continental glaciers swept across deep, that is separated from the main lake by the Door Canada and into the Superior basin, they were guided Peninsula (a western extension of the crescent-shaped by the valleys. The immediate cause of the present sub- cuesta known as the Niagara Escarpment that also sep- marine topography was deepening by successive lobes arates Georgian Bay from Lake Huron and Lake Erie of glacial ice that occupied the bottom of the rift zone, from Lake Ontario). thus eroding the softer sediments but modifying only Lake Michigan lies wholly within the Paleozoic to a moderate degree the resistant Precambrian rocks bedrock province. Rock exposures are not common of the sides. around the lake because glacial deposits mantle the As they retreated, the glaciers and glacial lakes cov- bedrock almost everywhere. However, exposures are ered the bottom surface with a thin layer of drift and sed- sufficient to show that Paleozoic rock formations con- iment. Irregularities and deep canyons in the western trol the major topographic feature of the lake basin. part of the lake basin were filled with sediment, yielding Silurian-aged Niagaran Dolomite forms the cuesta that a smooth bottom, whereas, depressions in the eastern bounds the lake on the west and north and dips under the part of the lake were not filled, leaving the many irreg- lake toward the center of the Michigan structural basin ular north-south submarine canyons and ridges. While centered under the Lower Peninsula of Michigan. Mid- the glacial lakes were receding and establishing a series dle Devonian-aged Traverse Group limestones form the of temporary levels, waves carved ancient lake terraces shore from the Straits of Mackinac to the headlands of into the shoreline resembling gigantic stair steps. Sev- Grand Traverse Bay. The shoreline of Lake Michigan eral channels have drained Lake Superior at different ranges from Paleozoic bedrock that forms precipitous times. Outcrops of sandstone in the St. Marys River, cliffs and glacial debris along the northern and west- the present outlet, form a natural weir that restricts the ern shores, to expansive sandy beaches with enormous discharge of the lake. The water level in Lake Superior, dune ridges on portions of the eastern and southern at an average elevation of 183 m above sea level, is now shores.