. Bulletin. Natural history; Natural history. 252 Illinois Natural History Survey Bulletin Vol. 33. Art. 3 has a small (800 km^) watershed for a lake with 500 km^ of surface area. Its enormous volume of 156 km^, with a retention time of about 700 years, pro- vides a large buffering effect for nutri- ents entering the lake from tributary streams draining the forests, roads, parking lots, golf courses, and building sites of its highly disturbed watershed. Due to the long residence time of the water most of the nutrients entering the lake remain there with little flushing action and only slow sed
. Bulletin. Natural history; Natural history. 252 Illinois Natural History Survey Bulletin Vol. 33. Art. 3 has a small (800 km^) watershed for a lake with 500 km^ of surface area. Its enormous volume of 156 km^, with a retention time of about 700 years, pro- vides a large buffering effect for nutri- ents entering the lake from tributary streams draining the forests, roads, parking lots, golf courses, and building sites of its highly disturbed watershed. Due to the long residence time of the water most of the nutrients entering the lake remain there with little flushing action and only slow sedimen- tation to reduce them. In addition to stream-borne nutrients and sediment, air pollution is now visible, and storms bring acid rain to the basin as well as an important, but as yet inadequately measured, dry fallout (Fig. 5). Located at the crest of the Sierra Nevada, Lake Tahoe was first recog- nized as a graben fault basin by Le Conte (1875). It was probably formed during the upthrust of the mountain range between 3 and 9 million years ago and has a morphometry similar to that of a giant bathtub. The 450-m con- tour line of depth is very close to shore around most of its perimeter. Impor- tant to the lake's trophic status is its relatively infertile watershed consist- ing largely of decomposed granite de- rived from the great Sierra Nevada batholith as well as some Pi'ecretaceous metamorphic rocks, Pliocene volcanics, and a few cinder cones from the Holo- cene (Hyne et al. 1972). Ice damming of the Truckee River outlet of the lake occurred during the last period of Pleis- tocene alpine glaciation. At that time the water in the lake reached levels as much as 150 m higher than it is today. Because of Lake Tahoe's extremely low productivity, which, since 1959, has ranged from to g C m"^ day\ the more conventional measures of fertility, such as oxygen depletion, chlorophyll a, and change in chemical composition, have had little meaning. Oxygen levels have rem
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