Stream sediment and nutrient loads in the Tahoe Basin--estimated vs monitored loads for TMDL "crediting".

Total maximum daily load (TMDL) programs utilize pollutant load reductions as the primary strategy to restore adversely affected waters of the USA. Accurate and defensible "crediting" for TMDL reductions of sediment and nutrients requires stream monitoring programs capable of quantitative assessment of soil erosivity and the "connectivity" between erosive areas and stream channels across the watershed. Using continuous (15-min) stream monitoring information from typical alpine, snowmelt-driven watersheds [Ward (2,521 ha), Blackwood (2,886 ha), and Homewood (260 ha, Homewood Mountain Resort--HMR) Creeks] on the west shore of the Lake Tahoe Basin, daily sediment (and nutrient for HMR) loads are determined and compared with those developed from estimated load-flow relationships developed from grab sampling data. Compared to the previously estimated sediment load-discharge relationships, measured curves were slightly below those estimated, though not significantly so at Blackwood and Ward Creeks in the period 1997-2002. Based on average daily flowrates determined from calibrated hydrologic modeling during the period 1994-2004, average daily flowrate frequency distributions per year are determined from which load reduction "crediting" towards TMDL targets can be evaluated. Despite seemingly similar estimated and measured sediment load-flow relationships, annual "estimated" loads exceeded those "measured" by about 40 % for Ward and Blackwood Creeks and over 300 times for HMR Creek. Similarly, though less dramatic, estimated annual nutrient loads at HMR Creek exceeded those measured by 1.7 and 6 times for total nitrogen and total phosphorus, respectively. Such results indicate that actual measured load-flow relationships are likely necessary for realistic quantitative and defensible TMDL crediting.