Transcript Lake Origins

Lake Origins
Water bodies may be classified by their
origin.
 Gradual or catastrophic geological events
form and/or destroy lakes, streams,
wetlands and estuaries.
 Lakes are ephemeral in a geologic sense.
The rate of aging can be measured.
Most lake basins are created by gradual events.
 glacial activity
 deformation of the earth’s crust
Rapid catastrophic geologic events
 earthquakes
 landslides (Mount St, Helens eruption
triggered massive mudslides expanding Spirit
Lake)
 volcanic eruptions
Lake districts
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Lakes formed in one geographic area are
generally created by some common
natural event.
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
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Beavers dam up streams, shallow but
extensive lakes.
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.
 Faulting: Lakes may be associated with the
movement of a single fault.
 depressions formed by tilting.
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Grabens are lakes associated with the
movement of multiple faults.
 Lake Tahoe: depressed area between
adjacent faults.
Tectonic formation
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The size of the lake depends on the
magnitude of the faulting and the amount
of silting over the years since formation.
Uplift of portions of sea floor created Lake
Okeechobee, Florida
 area = 1840 km2, the second largest
surface area of freshwater lake in the
United States.
Rift lakes: One great fault forms a series of
lakes.
 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).
 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.
 Differential cooling of lava forms lake basins:
 Yellowstone Lake in Yellowstone National
Park.
Glacially formed lakes
Glacial activity has been the most important lakecreating force over the last few millennia.
 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.
 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
Glacial moraines may dam a stream and form a lake.
Moraine 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-tovolume ratio
and the wind fetch is small.
Walden Pond in Massachusetts
Kettle Lake
Kettle lakes
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.