Spatial and temporal dynamics of watershed sediment delivery, Lake Tahoe, California.

摘要

Turbidometry, grab sampling and geomorphological approaches were applied to the study of spatial and temporal patterns of sediment delivery in Lake Tahoe watersheds. Ward and Blackwood Creeks had particularly high specific sediment loads for Lake Tahoe, but low loads relative to other regions. For the period 1982–2000, peak discharge had a higher correlation with annual suspended sediment load than other hydrologic variables. Peak discharge and maximum instantaneous loading rates were observed for rain-on-snow events. Clockwise hysteresis, a lag between sediment concentration peak and discharge peak, was observed for daily, seasonal and inter-annual scales. The hysteresis indicates sediment supply limitation, and may explain an observed overestimation of sediment loads calculated from rating curves as compared with turbidometry. Turbidometry proved to be an effective proxy for suspended sediment and total phosphorus loading but not dissolved phosphorus. Ward Creek badlands and similar regions in Blackwood Creek were major sediment sources, contributing 20–40% of the spring sediment load, while comprising only 1% of the watershed area. Main stem low-gradient reaches appeared to be operating as temporary storage for fine sediment, storing during low flow and releasing during high flows. Geomorphological measurements including: woody debris, ratio of fine sediment to total pool volume (V*), entrenchment ratio, pebble count, and stream bed stability, supported conclusions drawn from turbidometer and grab-sample methods. Bed stability was an effective tool for measuring watershed and stream channel condition. High pool sediment volumes (V*) corresponded with high sediment loading for two streams tributary to Ward and Blackwood Creeks. Comparison of pebble count data with literature values for spawning gravels suggests undesirable sedimentation of spawning gravels in lower reaches of Blackwood Creek and lack of suitably-sized spawning gravels in lower reaches of Ward Creek. Woody debris quantities were 10–20% of literature values for streams in undisturbed forest. Both watersheds have severe channel incision in lower reaches. Limiting peak flows by maintaining and improving water storage in soil, and riparian aquifers, and restoring regions degraded by grazing and logging, should result in large reductions in sediment loading and help to reverse the progressive eutrophication of Lake Tahoe.