Lake Origins Water bodies may be classified by their origin. n Gradual or catastrophic geological events form and/or destroy lakes, streams, wetlands and estuaries. n Lakes are ephemeral in a geologic sense. The rate of aging can be measured.

Most lake basins are created by gradual events. n glacial activity n deformation of the earth’s crust

Rapid catastrophic geologic events n earthquakes n landslides (Mount St, Helens eruption triggered massive mudslides expanding Spirit Lake) n volcanic eruptions

Lake districts n Lakes formed in one geographic area are generally created by some common natural event. n In a lake district the lakes have similar characteristics  but their water quality,  basin morphometry and biological productivity may differ.

Minnesota

Non-geologically formed lakes n Beavers dam up streams, shallow but extensive lakes. n Humans create artificial lakes  damming rivers and streams  irrigation  water storage  hydroelectric power generation

Tectonically formed lakes Shifts in the earth’s crust  uplifting of mountains  breaking and displacement of rock strata  cause part of a valley to sink  creating a depression that fills with water. n Faulting: Lakes may be associated with the movement of a single fault.  depressions formed by tilting.

n Grabens are lakes associated with the movement of multiple faults.  Lake Tahoe: depressed area between adjacent faults.

Tectonic formation n The size of the lake depends on the magnitude of the faulting and the amount of silting over the years since formation. n Uplift of portions of sea floor created Lake Okeechobee, Florida  area = 1840 km 2, the second largest surface area of freshwater lake in the United States.

Rift lakes: One great fault forms a series of lakes. n In eastern Africa=  Lakes Malawi, Tanganyika, Edward, Albert, and Turkana, and now marine Red Sea.

Volcanically formed lakes Worldwide distribution Calderas (broad craterlike basin of a volcano, formed by an explosion or by collapse of the cone). n Best known = Crater Lake, Oregon: 10 km across, >600 meters deep. Lava flows may dam a valley to form a lake:  Snag Lake, in Mount Lassen National Park, CA. n Differential cooling of lava forms lake basins:  Yellowstone Lake in Yellowstone National Park.

Glacially formed lakes Glacial activity has been the most important lake- creating force over the last few millennia. n Most of the world’s lake basins, including the Great Lakes,  were formed during the Pleistocene Era  when glaciers covered much of the earth.  Lakes filled with water as glaciers melted and shrunk. n Famous lake districts:  Great lakes, Wisconsin (land of a 1,000 lakes)  English and Scottish, Ontario, Canada  Scandinavian and Alpine Lakes (in general)

Quebec

Cirque lake (French cirque, meaning semicircle or amphitheater> Glacial rock-basin lake. Usually found at the head of glaciated valleys where the valley abuts the steep slope of a mountain. Deepest near the cliff and shallow near the outlet. The water is dammed at the outlet by a low barrier of glacial debris called a moraine.

Cirque lake

Moraine Lake Glacial moraines may dam a stream and form a lake.

Glacial valley lake

Rock is eroded by the slow downhill movement of the glacier aided by continual freezing and thawing activity that fractures the rock. A series of valley-rock-basin lakes like the beads on a rosary are called paternoster lakes.

Blocks of ice trapped in a glacial moraine, melts and a kettle lake is formed. Kettle lakes: are very steep sided, may be meromictic (the bottom water never mixes with surface water, because of their small surface-to- volume ratio and the wind fetch is small. Walden Pond in Massachusetts

Kettle Lake

Kettle lakes

Oxbow and scroll lakes are small lakes formed in the flood plains of rivers. Oxbow: formed when the loop of a meandering river is cut off by silt deposition. Found in the flood plains of almost any river worldwide.

Oxbow formation, erosion through meander neck

Scroll lakes: a former river channel has moved as a result of sediment deposition at a bend. Flood plain lakes are connected to the river during high floods, and become a great habitat for fish.