Topaz Lake Watershed

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Topaz Lake (36°37'30"N, 118°38'11"W) is at the head of the Tokopah Valley (the upper Marble Fork of the Kaweah River) in Sequoia National Park, approximately 6 km north-northwest of Emerald Lake. Research at this catchment began in 1986. The lake is shallow (mean depth 1.5 m), covering an area of 5.2 ha and with a volume of 76,900 m3. The lake is connected by a narrow channel to a small, shallow pond during high-water periods. Water in the pond is similar in chemical composition to Topaz Lake. During the seven years of record, the lake froze to depths of 1 to 2.9 meters, corresponding to 50-94% of the lake volume. In extreme winters, such as 1983, it is likely that the entire lake is composed of ice and slush. Thermal stratification of the lake occurs only in winter months. Low dissolved oxygen concentrations were measured during most winters; ice-free surface waters are always well oxygenated. Maximum summertime lake temperatures ranged from 14 to 19°C.

No fish were observed in Topaz Lake despite the lake being stocked with trout several times this century; the lake is probably too shallow to support fish. Trout exert a substantial influence on the zooplankton and zoobenthos of Sierran lakes. In contrast to lakes with fish, large zooplankters (Diaptomus eiseni, Daphnia middendorfianna) are plentiful in Topaz Lake. Callisbaetis, hemipterans and dytiscid beetles (Hydroporus) were also abundant relative to populations in other lakes and ponds within the Tokopah Valley.

Vertical relief in the basin is 275 meters and the watershed has a southern exposure. Parts of the upper basin have extensive meadows (grasses and sedges) and short-lived ponds during snowmelt. There is a small stand of Foxtail Pines in the upper eastern portion of the watershed (~25 trees). Alpine Brown Soils are found throughout the watershed, often forming complexes with rocks and bedrock outcrops. Extensive wet meadows are found along the north shore of Topaz Lake and around other ponds in the basin. The geology of the basin is dominated by fine-grained, porphyritic granodiorite containing abundant mafic inclusions. The phenocrysts include potassium feldspar, hornblende, biotite and plagioclase. Because of the gentle relief surrounding it, the lake expands during snowmelt, flooding a small meadow along the northern shore and forming a large bay. The bay comprises a substantial portion of the lake's area, although not its volume. As summer progresses and lake level declines, water retreats from the bay. Besides the lake and the shallow pond, other surface waters in the catchment are short-lived. During drought years, the water level in the lake drops by as much as a meter below the outlet.