Analysis of Lake Tahoe Interagency Monitoring Program (LTIMP) data (sixteen years, nine streams) indicates that 75% of annual stream phosphorus (P) loading occurs during spring snowmelt. Intra- and inter-stream annual stream total P (TP) and phosphate (PO4) loads can vary up to three and five orders of magnitude, respectively. Blackwood Creek, Trout Creek, Upper Truckee RIver, and Ward Creek deliver 80% of LTIMP stream annual loads. The Upper Truckee River is responsible for 35% of these P loads. Discharge, P load, precipitation, basin area, basin slope, and impermeable coverage are all correlated with one another, making the statistical linkage of human disturbance and stream P at the watershed scale unlikely. It is probable that humans are negatively impacting stream P in an ecological manner and that an important watershed parameter was not examined in this study.

Particulate P (PP) is the major form of P in Tahoe streams, comprising 45-88% of annual TP concentrations. Relatively P-poor sand-sized particles dominate stream P transport during high-discharge events; relatively P-rich silt- and clay-sized particles dominate during low-discharge conditions. Dissolved organic P comprises 6-42% of annual TP concentrations, while PO4 comprises 6-19% of annual TP concentrations in LTIMP streams. Hydrological events (rain-on-snow, rain-on-soil, spring snowmelt) produce the highest P concentrations, but streams with the highest annual P concentrations do not generally have the highest annual loads. Suspended sediment, TP, and PP concentrations are correlated with geologic surficial deposits, indicating that stream sediment and PP is coming from near-stream sources.

Algal bioassay work indicates that PO4 best characterizes short-term biologically-available P from Tahoe tributaries, with 75-90% of lake algal response (6 day incubation) to stream water coming from P in the sub-0.45 micron range.

Human development will likely have the greatest impact on stream P during high precipitation years. Reduction of the erosive power and P content of urban runoff during the spring snowmelt season is recommended to reduce human impacts on Tahoe stream P the P-limited Lake Tahoe.