Turbidity is a measure of water’s clarity, and this standard establishes a goal for the nearshore. The nearshore represents an important socio-economic value because this is where most visitors and residents experience the lake first-hand. Turbidity in the nearshore is influenced by many factors including wave action, algae, and discharge from streams, especially during storms. Projects and programs reduce pollutant and sediment loading to the lake and also are likely to improve nearshore clarity.  

 

 

Status
Nearshore Turbidity.JPG
Distribution of section average values obtained from individual nearshore clarity circuits at a seven meter depth (23 ft.) completed using equivalent methods since 2001. Survey re listed in chronological order on the x-axis (mm/yyyy format). Survey dates are non-continuous. The blue line follows median values from each circuit represented by quartile boxplots, with whiskers to outermost data points within 1.5 times the interquartile range.
2019 Evaluation
See how thresholds are evaluated
Status
At or Somewhat Better Than Target
Trend
Little or No Change
Confidence
Low
Applicable Standard
WQ4: Turbidity shall not exceed one NTU in shallow waters of the Lake not directly influenced by stream discharges.
Key Points
  • Tahoe's nearshore waters, similar to its deep lake waters, are incredibly clear. Scientists have suggested the need to better understand the drivers of nearshore clarity. Recent investment by TRPA and partners has focused on improving our understanding to both target management to improve clarity and identify appropriate goals for nearshore clarity.
  • In 2014, UC Davis began deploying a network of sensors that continuously monitor nearshore conditions (chlorophyll, dissolved organic matter, wave height, temperature, conductivity, and dissolved oxygen). Preliminary findings suggest that nearshore clarity is primarily influenced by wind and wave height.
  • During high wind events, nearshore sediments are resuspended in the water column, and decrease the clarity of the water, until the wind speed and wave height are reduced.
About the Threshold
Water clarity refers to the transparency or clearness of the water and is a commonly used indicator for the health of a water body. Federal, state, and regional agencies have adopted regulations to protect Lake Tahoe’s renowned clarity, which includes both the pelagic (deep water) and the littoral (nearshore) zones. The nearshore represents an important socio-economic value because this is where most visitors and residents experience the lake first-hand. Both California and Nevada recognize the unique ecological and aesthetic values of the nearshore environment, and both have adopted standards to protect nearshore water clarity. 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 indicators of nearshore clarity.
Nearshore turbidity is primarily driven by the concentration and type of fine particulate materials suspended in water. Nutrient loading affects clarity by increasing phytoplankton growth (suspended organic particles). Suspended sediment loading affects clarity by contributing more fine inorganic particles. Both types of particulates cause nearshore clarity loss by scattering light and through light absorption. Heterogeneity of nearshore features leads to considerable variability in effects from environmental and anthropogenic drivers. Main drivers include seasonal runoff and lake water-column mixing, as well as episodic storm runoff and localized upwelling events. Deep-water zones close to the shoreline may mitigate the intensity of these effects by mixing offshore water with nearshore water and diluting clarity-reducing constituents. Extended shallow-water shelves can accentuate impacts by retaining higher concentrations of nutrients and suspended sediment particles, as well as by providing warmer water conditions for increased biological activity and sediment resuspension from waves and boat wakes. Urban stormwater runoff generally contains much higher concentrations of nutrients and fine sediment particles than found in the lake or in runoff from undisturbed areas. Urban stormwater discharges to the lake generally derive from impervious areas that include transportation routes and associated conveyance systems. They cause locally elevated concentrations of phytoplankton and suspended fine sediment particles that contribute to diminished nearshore clarity. Stream water concentrations of nutrients and sediments are naturally higher than pelagic lake water, so even streams from undisturbed watersheds contribute fine sediment particles and nutrients. Streams that pass through disturbed watersheds, however, contribute significantly higher concentrations of nutrients and fine sediment particles than streams from undisturbed watersheds. Inputs from groundwater seepage directly into the lake can increase concentrations of dissolved nutrients (e.g., nitrogen and phosphorus), which increase concentrations of the microscopic suspended algae that decrease nearshore clarity. Upwelling events and seasonal lake mixing deliver deep-lake waters to the nearshore. These waters often can be nutrient-rich relative to nearshore conditions. Accumulated fine sediments that have settled in the nearshore may have an impact on transparency during times of high winds or when spring snowmelt increases lake levels and exposes the newly submerged land surface to wave action. The cumulative effect of boat wakes during peak recreation periods can induce episodic sediment resuspension in the nearshore. Atmospheric deposition of fine sediments and adsorbed nutrients from road dust can have a disproportionately greater effect on the nearshore compared to deep lake sites due to proximity. Long-term climate trends are likely to impact nearshore conditions as more precipitation arrives through rain rather than snow, with higher contributions of nutrients and fine sediments at some locations from increased runoff scouring of the landscape. Higher surface water temperatures from warming climate conditions may also increase nutrient cycling and phytoplankton production.
Delivering and Measuring Success

No related projects or programs defined for this indicator.

Rationale Details
Wind and wave action are the primary causes of periods of high turbidity when the target established by the standard are exceeded. No formal method has been adopted for spatially delineating the zones established in the standard: areas not directly influenced by stream discharge and areas directly influenced by stream discharge.
There is no evidence to suggest changes in nearshore clarity have occurred over the last four years.
Confidence Details
Additional Figures and Resources

No photos available.


No documents available.