Download Lecture 9

Cirque lakes in the rockies
•Glaciers in headwater valleys tend to
scour out a bowl shaped basin and the
excavated material forms a moraine at
the lake outflow that maintains the lake
level after the glacier has receded.
•Drainage in Moraine lake was further
impeded by a large landslide across the
outflow
•Most cirque lakes are fishless unless
stocked
Pothole or kettle lake formed in glacial --usually small < 30 ha, but can be
quite deep--10-40 m. Watersheds are very small.
•Large blocks of ice left behind in moraines and till mounds as glaciers melt
and grow “stagnant”.
•As they gradually melt, they leave behind a depression in the till that fills by
seepage
•Many of the small pothole lakes in Alberta are kettle lakes.
Another type of basin associated with ice melting.
Polygonal ponds
near the Lena
River, Russia
Polygon ponds form along the Arctic coastal lowlands.
Form in the summer as wedges of ice melt within the permafrost to form small
polygonal basins (around 50 m across) that fill up with surface water.
See Fig. 6.2 in your text
Most of the large and old lakes in the world are
Tectonic lakes
Many occupy ancient basins called grabens—formed by large geological faults
Rocks before faulting
Lake in a symmetrical graben
Lake in a tilted graben
Lake Baikal—one of the most famous tectonic lakes in the world, has existed for
over 20 million years
Crater Lake, Oregon -589 m deep and possibly the clearest lake in the world,
Transparency up to 90 m.
Thermocline very deep for its size
No rooted plants.
Mud doesn’t accumulate on the bottom till > 90 m depth
Why is this
lake so
different
from most
lakes?
Some of the most spectacular tectonic lakes are formed in volcanic craters.
Physical features of lakes that determine habitat characteristics
•inflow from the watershed/Catchment
•Water residence time
•Morphometry, Mean depth and volume
•Thermal stratification and physical mixing
•wind./currents/wave action
•Sediment deposition
•Light extinction