Nearshore Clarity Report

No Project associated with this Finding

Finding Details

This project was implemented to demonstrate a pilot monitoring program for evaluation of nearshore clarity metrics identified by the Lake Tahoe Nearshore Monitoring and Evaluation Framework. That framework recommended a minimum of four contiguous nearshore surveys each year to assess spatiotemporal patterns in turbidity, light transmissivity, and chlorophyll-a fluorescence.

Nearshore turbidity is used as an existing TRPA threshold standard, which requires that measurements show less than 1 NTU (or less than 3 NTU in regions of stream influence). No single value of turbidity measured along the approximately seven-meter depth contour was greater than 0.999 NTU. The lowest single turbidity value measured was less than 0.081 NTU. These measurements were necessarily conducted during non-storm periods, however, so nearshore monitoring closer to outfall points during storm runoff events would likely yield values exceeding 1 NTU.

Spatial patterns in nearshore turbidity were evaluated by plotting interquartile ranges from the five monitoring circuits by distance sequentially around the nearshore. Regionally, though not consistently, nearshore turbidity was found to range highest near urban areas along the south shore, the northeast shore and the northwest shore of Lake Tahoe.

Transmissivity measurements were moderately correlated with turbidity (r = -0.43), and spatial patterns were similar, with the exception that the west shore of the lake showed more variability and lower clarity conditions than were assessed from turbidity measurements.

Chlorophyll fluorescence was weakly correlated with transmissivity (r = 0.02), but there was no apparent direct correlation with turbidity.

A regression of watershed cumulative percent impervious cover density versus average turbidity over corresponding nearshore sections was relatively weak (R2 = 0.214). Further examination of this relationship, including better delineation of landscape features and nearshore factors is recommended. This may benefit from development of models of the nearshore that incorporate important processes, such as hydrodynamic patterns, and localized features, such as stream inputs, stormwater outfalls and substrate composition.