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Secchi disk transparency is measured in the pelagic zone of Lake Tahoe, but this approach does not work in the littoral (nearshore) zone where water depth is insufficient for the method. Instead, instrument measurements of turbidity and light transmissivity are used as metrics of nearshore clarity.
No single turbidity measurement taken in recent lake-wide circuits exceeded the existing TRPA threshold standard of one nephelometric turbidity unit (NTU).
The 2017 State of the Lake Report reviewed newly-gathered continuous turbidity measurements in relation to physical processes, and noted that measured elevated turbidity was often correlated with elevated wave height.
Nearshore Turbidity Monitoring
A pilot monitoring program of nearshore turbidity began with the first circuit completed in November 2014, followed by similar nearshore circuits completed in April, June, August and November 2015. Measurements were made at a depth of seven meters. Routine boat operating speeds are typically around 10 kilometers per hour in the nearshore areas (Heyvaert et al., 2016). Beginning 1991, nearshore turbidity was measured offshore at the 25-meter depth contour for several locations, including 1) mouth of Upper Truckee River and Trout Creek; 2) El Dorado Beach; 3) mouth of Edgewood Creek; 4) Nevada Beach; 5) mouth of Incline Creek; 6) Burnt Cedar Beach; 7) mouth of Ward Creek; 8) Tahoe State Recreation Area; and 9) the mouth of Blackwood Creek. More recently, nearshore clarity has been measured at approximately the seven-meter contour following a continuous circuit around the lakeshore. This strategy is considered more representative of littoral conditions where people interact with the lake