Lakes of the Cordillera

The cordilleran system of North America, extending from the Brooks Ranges and Aleutian peninsula in Alaska to the isthmus of Panama, is home to a great many natural lakes. Two processes are responsible for the formation of most of these: glaciation and tectonics. The glacial lakes are mainly in the northern half of the cordillera, where Pleistocene ice was most extensive. Lakes of tectonic origin are found throughout the region, although in the north most of these are also modified by glacial action. In the more arid portions of the region the existence of lakes is closely tied to precipitation and runoff, and many lakes have no outlets to the sea.

Glacial lakes Lakes are common in mountainous regions affected by glaciation. Most of them are formed by erosion by glaciers flowing in valleys, and many are impounded by terminal moraines forming natural dams. They tend to have steep shorelines and many are quite deep. At one time they received melt-water from glaciers and some still do, forming deltas at the up-valley ends of the lakes and blanketing lake beds with layers of sediment.

The largest of the glacial lakes in the cordilleran system are found in the Canadian Rockies. Not only is this region geologically conducive to lake formation by virtue of numerous sub-parallel mountain ranges, but climatically it is an area of considerable snowfall and thus prone to glaciation. Most of the glacial lakes in the Canadian Rockies occupy structural troughs formed by faulting and enlarged by glacial action. Among the largest of these are Williston Lake, Ootsa lake, Kootenay Lake and Okanagan Lake in British Columbia, and Tagish Lake, Atlin Lake, and Lake Bennett spanning the British Columbia-Yukon border.

Thousands of smaller glacial lakes are found throughout the Rockies, generally sharing the characteristics of being deep and often long and narrow. The smallest of the lakes formed by glacial erosion occupy small bowl-shaped hollows called cirques. These are found high in the mountains and thus these lakes are typically ice-covered most of the year.

Tectonic basins The Cordilleran system is a region is active mountain-building, and as such there are many areas with considerable recent tectonic activity. This inevitably results in the formation of topographic depressions that, if climate permits, become filled with water. These depressions can be very large, both in terms of area and depth. For example, at the southern end of the Cordillera Lake Nicaragua and Lake Managua lie adjacent to each other at the bottom of a tectonic depression between two mountain ranges. They are relatively low in elevation (32 and 37m, respectively). Lake Nicaragua, with an area of over 8000 km2, is the 9th largest in North America. Lake Nicaragua was once connected to the ocean and thus contains many marine species not found in inland lakes. Lake Chapala, the largest lake in Mexico, lies in a tectonic basin south of Guadalajara. Increasing demands for fresh water in the region have had significant impacts on the lake. Satellite imagery shows that the lake surface area decreased from 1048 to 812 km2 between 1986 and 2001, corresponding to a water level decrease of 2-4 m.

Active volcanism also contributes to formation of lake basins, such as Crater Lake in Oregon (a volcanic caldera) and Lake Catemaco in Mexico, (a basin dammed by a lava flow).

Endorheic lakes In the drier parts of the Cordillera the existence of lakes is dependent on climatic conditions as well as topography. If evaporation rates are high enough to equal the amount of runoff and precipitation entering the lake, then there may be no surface outflow. Such lakes are called endorheic. They tend to have high concentrations of dissolved solids as a result of evaporation, and concentrations fluctuate with water level, as higher water volumes dilute dissolved solids (refer to 'see also' section). Great Salt Lake is the best-known example in North America. Other prominent internally-draining lakes in the western United States include Pyramid Lake in Nevada, and Mono Lake in California. The lakes around which the Aztec culture flourished in the Valley of Mexico were also endorheic. They have since been drained and replaced by Mexico City.

Because there is no overflow control, the levels of endorheic lakes fluctuate substantially in response to climatic variations, rising in wet and/or cool periods and falling in dry and/or warm periods. During the Pleistocene, reduced temperatures cause large reductions in evaporation rates so that the level of water in what is now Great Salt Lake rose by more than 300 m, and the lake grew in area to more than 50 000 km2, to form Lake Bonneville. This was the largest of several lakes known as pluvial lakes, associated with wetter periods of the Pleistocene in areas that were not glaciated. Lake Bonneville's maximum area was about ten times the present area of Great Salt Lake. At that maximum extent it overflowed to the north, into the headwaters of the Snake River. Another great pluvial lake called Lake Lahontan occupied several valleys in western Nevada and Eastern California, but did not overflow to the sea. Variations in levels of these prehistoric lakes caused massive floods whenever lake level rose to the elevation of a key overflow point, and their marks on the landscape are prominent today.

The shallowest endorheic lakes are ephemeral: they only contain water following brief periods of high runoff and disappear by evaporation between such periods. These are called playas, and are common in the intermountain western United States and northern Mexico. Many of these never disappear completely and at their smallest extent sustain populations of salt-tolerant organisms such as brine shrimp.

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