Physiography of Lakes and Reservoirs - Freshwater Ecology: Concepts and Environmental Applications - page 99

prairies of North America, although many have been filled for agricultural
purposes. If the forward flow of a glacier is approximately equal to its
backward melting rate, a wall of material is formed called a terminal
moraine, which can impound water flow and lead to formation of lakes.
Materials deposited along the sides of glaciers form lateral moraines.
Glacial lakes tend to be smaller than tectonic lakes, but a few very large
glacial lakes (e.g., the North American Great Lakes) make up a consider-
able area when considered on a global basis (Fig. 6.5).
A catastrophic mode of lake formation is the release of large volumes
of water from behind glacial ice dams. Some of these outbursts happen on
a moderate scale now; larger ones occurred during the last ice age. These
outbursts occurred as a result of pooling of glacial water as large ice sheets
receded, followed by collapse of the ice dam. Such outbursts created mas-
sive floods that scoured out existing lakes and created new lakes below any
spillways that existed in the channels. Kehew and Lord (1987) suggest that
such outbursts established the courses of most major rivers in the midcon-
tinental United States and Canada. Lake Missoula was formed in western
Montana behind the retreating ice sheet and
was responsible for several massive floods
downstream in the Columbia River basin.
The lakes in the Grand Coulee in eastern
Washington state are below the spillways
where the floods dug out massive quantities
of sediment and deeply incised the channels.
Volcanic activities can lead to formation
of lakes. Explosions of volcanoes or pockets
of steam can leave behind depressions in the
craters that fill with water. These lakes are
called caldera or maar lakes. An example of
a volcanic lake is the exceptionally clear and
deep Crater Lake in Oregon (Fig. 1.1). The
lake was formed when the volcano Mount
Mazama exploded about 6000 years ago. This
eruption must have been a catastrophic event
for Native Americans living in the region dur-
ing the time since several meters of ash were
deposited throughout western North Amer-
ica. The resulting crater filled with water and
a subsequent eruption formed a volcanic cone
in the lake known as Wizard Island.
Water can dissolve sedimentary rocks
and lead to depressions that form lakes. In
karst regions, sinkholes form where lime-
stone is dissolved and the cavity collapses to
form a lake. Similar processes can occur
where old subterranean salt deposits are dis-
solved or where sandstone is washed away.
These dissolution lakes are generally small.
Activities of rivers can form fluvial lakes,
including oxbow lakes where meanders

Sidebar 6.1.
A Large Lake beneath the Ice
in Antarctica

In 1974 and 1975, an airborne radio-echo sur-
vey of Antarctic ice depths led to the discov-
ery of a lake under the ice. The ice sitting on
the lake's surface is flat relative to the sur-
rounding ice sitting on land, and remote satel-
lite measurements of ice elevation have al-
lowed determination of the size of the lake
(Kapitsa et al., 1996). The lake is estimated to
cover about 15,000 km 2 , is 125 m deep, and
rests below about 4 km of ice. Preliminary cal-
culations suggest that the residence time of
the water in the lake is tens of thousands of
years, and that the lake basin is about 1 million
years old. However, there is significant water
exchange between the lake and the ice sheet
(Siegert et al., 2000). Scientists have taken
cores through the ice sheet to 3950-m depth
(about 120 m above the lake). The ice at 3310
m is about 420,000 years old and was formed
by refrozen lake water (Jouzel et al., 1999).
Analyses of the refrozen lake water from the
ice cores indicate the presence of a microbial
community (Priscu et al. 1999, Karl et al., 1999)
and some of these bacteria may still be viable
(Karl et al., 1999). Sampling the lake without
contaminating it will be technically difficult but
is certain to yield interesting results.