Glaciated interior plains In the northern, glaciated, interior plains, the Pleistocene glaciers originating in the Canadian Shield flowed westward to the base of the Rocky Mountains. There they met ice flowing eastward out of the Rockies. Thus, the entire width of the plains was glaciated for most of the extent of the Cordillera in Canada. Lakes are much less abundant on the glaciated interior plains than in the Shield. Lake Claire (1415 km2) and Lesser Slave Lake (1167 km2), both in northern Alberta, are the largest natural lakes of the region. In northeastern North and South Dakota as well as in much of Manitoba and Saskatchewan a great many small lakes, known as prairie potholes, exist. These are small lakes formed in glacial depressions; some are merely low spots in an undulating surface while others are kettle lakes. They are particularly significant as habitat for migratory waterfowl. In semiarid portions of the plains numerous endorheic saline lakes exist, particularly in Saskatchewan and Alberta (refer to 'see also' section).
Unglaciated interior plains The southern, ungla-ciated portion of the interior plains is essentially without natural lakes. The absence of lakes is a result of the lack of a lake-forming process such as glaciation or tectonic activity. Some of the major rivers have substantial oxbow lakes, formed by abandonment of a portion of channel by an actively meandering river. There is a high density of natural playa lakes in the Texas panhandle and adjacent New Mexico. In this area over 20 000 small endorheic ephemeral lakes dot the prairie. The depressions were formed by wind erosion, and standing water is maintained by clay-rich sediments that limit percolation losses. They grow and shrink seasonally, and form important islands of biodiversity in an intensively agricultural landscape.
Southeastern U.S. The portion of the United States south of the Pleistocene glacial limit and east of the Great Plains has very few natural lakes, although as will be discussed below there are millions of reservoirs. Nearly all of the natural lakes that are present are of one of two types: oxbow lakes and karst sinkholes that intercept groundwater. The largest oxbow lakes are found in the lower Mississippi valley. In this area the channel of the Mississippi is generally 1-2 km wide, and meanders actively although artificial controls on the channel have reduced this activity in the last century. The oxbow lakes of the lower Mississippi are typically 1-2 km wide several km long, and generally not more than a few meters deep. Reelfoot Lake, lying in the Mississippi flood-plain in northwest Tennessee, is thought to have been formed at least in part by the New Madrid earthquakes of 1811-1812. Smaller rivers draining the coastal plain also have scattered oxbow lakes on their floodplains, of smaller dimensions appropriate to the rivers that created them.
The Florida Peninsula contains the highest concentration of natural lakes outside the glaciated portion of North America. Lake Okeechobee is the largest (1890 km2). These lakes are generally small, circular depressions formed by underground solution weathering of limestone bedrock and subsequent collapse, forming sinkholes. Landscapes with such features are called karst, and are common in the southeastern United States. In most U.S. karst areas the sinkholes do not reach the ground water table, and so do not form lakes. In Florida, however, a great many sinkholes were formed during periods of the Pleistocene when sea level was substantially lower than at present. As sea level rose so did water tables and these sinkholes became lakes. Lakes so formed are also known as cenotes. Some are relatively shallow, but many are quite deep and connected to underwater cave systems.
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