Influences on water clarity near the Upper Truckee River Outlet in Lake Tahoe, California-Nevada

Understanding variability in near-shore water quality is an important step to protecting the integrity of Lake Tahoe's exceptional clarity. This study examines the near-shore area adjacent to the largest tributary to Lake Tahoe, the Upper Truckee River as conditions change over the course of a year. Changes in turbidity are compared to water temperatures, streamflow, wind and weather conditions between October 2002 to August 2003. Turbidity is observed to be generally low in the fall and winter, when conditions are calm and stream inflow is lowest. Isolated areas of elevated turbidity are caused by storm events accompanied by precipitation and wind. During the spring, greatly increased streamflow and stratification of lake water combine to create widespread elevated turbidity. After the spring snowmelt period, turbidity decreases slowly during calm summer conditions. Analysis of filtered water samples collected at the mouth of the Upper Truckee River shows that mineral particles are largely absent from near-shore samples during calm fall and winter conditions, but comprise a large portion of the particle composition following storm events during all times of the year. An overall increase in particles during the spring runoff and summer samples is observed. Samples of surface water collected 2.5 km off-shore display a general absence of particles of all sizes. Drifter experiments show that calm summer conditions produce currents sufficient to flush inflows from the near-shore area into deeper areas on a daily basis.