Transcript 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