Since the late 1980’s, USGS has collected discharge, sediment, and water quality data at seven major drainages as part of the Lake Tahoe Interagency Monitoring Program (LTIMP). Continuous, real-time measurements of turbidity recently were added to LTIMP sites. Similarly, the U.S. Department of Agriculture, National Resources Conservation Service (NRCS) has conducted continuous snowpack and soil monitoring in basin headwaters, with daily snow measurements dating back to the late 1970’s or early 1980’s at most sites. These data can be combined with remotely sensed datasets available from USGS and NASA and analyzed to determine the key factors controlling measured fine sediment and nutrient load in LTIMP streams draining to Lake Tahoe.

Lake Tahoe Basin data can now be accessed through the Lake Tahoe HydroMapper. The HydroMapper is an interactive map viewer which allows users to see real-time information on stream flow discharge, stage, nutrient, turbidity, sediment loads, and storage data. Data from NRCS Snow Telemetry (SNOTEL), National Weather Service Advanced Hydrologic Prediction Service and other local and regional hydrologic data with weather radar, watershed extents, and other ancillary geospatial data are included.

In addition to the data available in the HydroMapper, hydrologic functioning of the seven LTIMP watersheds are be assessed by determining key hydrologic conditions that drive daily variability of instream water quality at daily, monthly, and annual time intervals. The drivers may be a combination of dynamic (e.g., snowpack level, antecedent soil moisture or temperature, meteorological conditions) and static (e.g., terrain, geology, soil/vegetation) conditions within each watershed as well as various urban impacts and best management practices. Data about these key hydrologic conditions are available through this website.

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Lake Tahoe is a beautiful Alpine Lake in the Sierra Nevada. Known for its deep, clear water, Lake Tahoe is the second deepest lake in the United States, with a maximum depth of 1,645 feet. The average depth of Lake Tahoe is 1,000 feet.

Lake Tahoe Hydromapper

Open Full-screen View of the Lake Tahoe Hydromapper

Key Characteristics

Lake Elevation

Lake elevation is calculated by adding the gage height at USGS streamgage 10337000 to 6,220 ft. The natural rim of Lake Tahoe is 6,223 ft and the maximum legal limit is 6,229.1 ft (Bureau of Reclamation datum).

Lake Clarity

Lake Tahoe has the clearest water of any large lake worldwide. Lake clarity is measured using a method called the Secchi depth: a 8-10 inch white Secchi disk is lowered into the water and the deepest point at which the disk can be seen is the known as the Secchi depth. In 2019, the Tahoe Environmental Research Center (TERC) reported the annual average Secchi depth of Lake Tahoe to be 62.7 feet. This average is an 8.2-foot decrease from the prior year.

Lake Tahoe clarity had been declining by nearly a foot a year for nearly five decades. Annual clarity is improving but additional improvements are needed to get to the clarity restoration goal of 97.4 feet set by the Lake Tahoe Total Maximum Daily Load. Many factors contribute to lake clarity including algal growth, precipitation, sediment, lake warming, and lake mixing.

Secchi disk depth measurement in Lake Tahoe from University of California, Davis Research Vessel. Photos courtesy of Timothy Rowe, USGS.

Annual Average Secchi Depth

Annual data from University of California, Davis Tahoe Environmental Research Center.
Watanabe, S. and G. Schladow. 2023. Lake Tahoe Historic Secchi depth ver 3. Environmental Data Initiative. https://doi.org/10.6073/pasta/651f53d1340fc0131e05274672762f63. (Accessed 2023-04-11).

Recent Clarity Data

Fall, October-November; Winter, December-March; Spring, April-May; Summer, June-September.

Precipitation and Snow Water Equivalent

These graphs are created by the NRCS National Water and Climate Center.

Data

Current LTIMP Monitoring Sites

USGS scientists are collecting streamflow and water-quality data at Lake Tahoe tributaries as part of LTIMP’s commitment to providing long-term, consistent, reliable, and accessible streamflow and water-quality data. Together with the University of California, Davis, USGS has collected critical tributary nutrient and sediment data since 1988.

USGS Site ID USGS Site Name Drainage Area (m2) Year Data Collection Started
Discharge Stream Temperature Turbidity Sediment and nutrient monitoring
10336610 UPPER TRUCKEE RV AT SOUTH LAKE TAHOE, CA 54.9 1971 1981 2014 1980
10336645 GENERAL C NR MEEKS BAY CA 7.44 1980 1980 2016 1980
10336660 BLACKWOOD C NR TAHOE CITY CA 11.2 1960 1980 2016 1980
10336676 WARD C AT HWY 89 NR TAHOE PINES CA 9.70 1972 1980 2016 1980
10336780 TROUT CK NR TAHOE VALLEY, CA 36.7 1960 1981 2016 1980
10336698 THIRD CK NR CRYSTAL BAY, NV 6.05 1969 1980 2019 1980
10336700 INCLINE CK NR CRYSTAL BAY, NV 6.74 1969 2019 2019 1980

Compiled Data

Data from the current LTIMP sites have been compiled for the period of record covered by the LTIMP monitoring (approximately 1980 – present). Data analysis will assess the importance of watershed factors contributing to more recent instream continuous turbidity measurements using a statistical (e.g. machine learning) approach. Compiled data is available from the links below:

USGS Site ID USGS Site Name
10336610 UPPER TRUCKEE RV AT SOUTH LAKE TAHOE, CA
10336645 GENERAL C NR MEEKS BAY CA
10336660 BLACKWOOD C NR TAHOE CITY CA
10336676 WARD C AT HWY 89 NR TAHOE PINES CA
10336780 TROUT CK NR TAHOE VALLEY, CA
10336698 THIRD CK NR CRYSTAL BAY, NV
10336700 INCLINE CK NR CRYSTAL BAY, NV

USGS scientists collecting samples for the Lake Tahoe Interagency Monitoring Program. From left to right: Greg Hilgendorf, Nancy Alvarez, Tim Rowe.

Current Research

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Fine Sediment Tributary Loads

Using historical LTIMP and real-time monitoring data from USGS and SNOTEL sites, these questions are being addressed:

How do watershed processes related to snowmelt and runoff timing influence FPS and nutrients delivered to Lake Tahoe?

What hydrologic drivers control FPS/nutrient flux at the daily, monthly, and seasonal time scale at LTIMP sites?

A machine learning approach will be applied to existing data mined from USGS, USFS, NRCS, and remotely sensed products to extend and evaluate long-term trends and dynamics of FSP discharge and nutrient flux from Lake Tahoe watersheds.

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Tahoe Science Advisory Council (TSAC)

The USGS is actively participating in the multi-agency, bi-state TSAC to address science questions such as:

• What are the causes of changing trends in summer and winter lake clarity?

• What changes need to be made to reduce uncertainty in the current Lake Tahoe clarity model?

• Do existing water-quality standards for Lake Tahoe overlap, and how can these standards be streamlined?

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Assessment of Nutrient Sources using Stable Isotopes

High concentrations of phosphorus and nitrogen are responsible for excessive, or nuisance algal blooms in many ecosystems world-wide, and climate change is predicted to exacerbate the problem. Recent changes in periphyton biomass in the nearshore zone of Lake Tahoe may indicate changes in nutrient supply from human sources. Therefore, management actions that serve to limit external contributions of nutrients to the watershed will become even more important to Lake Tahoe in the future. The USGS is researching the sources of nitrogen and phosphorus nutrients in groundwater and in lake periphyton.

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Trends in Nitrogen, Phosphorus, and Sediment Concentrations and Loads In Streams

Lake Tahoe has 63 tributaries that are sources of nutrients and sediment to the lake. The lake’s clarity has been diminishing due to algae and fine sediment, prompting development of management plans. To understand the relative importance of land use, climate, forest management, and other factors affecting trends in nutrient stream concentrations and loads, the USGS developed a Weighted Regression on Time Discharge and Season model to simulate trends over time.