Suspended Sediment Concentration (Tributaries)
This indicator measures suspended sediment concentrations in Lake Tahoe's tributaries. Sediment (particularly fine sediment) delivered to Lake Tahoe is known to directly affect the transparency of Lake Tahoe. Suspended sediment concentration is the amount of organic and inorganic particles suspended in water in the stream. Excessive amounts of suspended sediment are the leading cause of impairment in streams and lakes of the United States. Landscape disturbances including, but not limited to, impervious road and parking lot surfaces, residential and commercial development, wildfire, and the degradation of stream environment zones, can contribute to sediment inputs to the lake and its tributaries. Suspended sediment concentrations are monitored in Lake Tahoe's tributaries through the Lake Tahoe Interagency Monitoring Program (LTIMP). Suspended sediment in Lake Tahoe focuses primarily on the fine sediments, those that measure less than 16 microns in diameter, about a fifth of the width of human hair.
Tahoe Stream Monitoring. Photo Credit: David Smith - USGS
Status
Average annual suspended sediment concentration in tahoe tributaries.
Evaluation Map
Water Quality Monitoring Locations
2023 Evaluation
- Status
- Considerably Better Than Target
- Trend
- Moderate Improvement
- Confidence
- High
Applicable Standard
WQ18: Attain a 90th percentile value for suspended sediment concentration of 60 milligrams per liter (mg/L).
Key Points
- Domagalski et al., 2021 found long term downward trends for both suspended sediment concentration and loads since the USGS began routine measurements in 1969.
- Excess concentrations of inorganic fine sediment particles in Lake Tahoe are responsible for the majority of the observed clarity loss.
- Following the extreme precipitation and hydrologic events of 2017 there has been an increase in fine sediment (0.5–16μm) concentrations in Tahoe’s tributaries. A similar increase has not been observed for overall suspended sediments (Smith 2022). Fine sediment particles (0.5–16μm) are primarily responsible for clarity loss in Lake Tahoe.
- Real-time data on stream conditions is available via the USGS Hydromapper
About the Threshold
- Sediment (particularly fine sediment) delivered to Lake Tahoe is known to directly affect the transparency of Lake Tahoe. Suspended sediment concentration is the amount of organic and inorganic particles suspended in water in the stream. Suspended sediments occur naturally in streams and are essential to the ecological function of a stream. Deposition of suspended sediments can create micro-habitats such as pools and sand bars for biota. Excessive amounts of suspended sediment are the leading cause of impairment in streams and lakes of the United States. Too much suspended sediment in the stream can have direct impacts on stream and lake biota. Identified impacts include loss of spawning habitat for fish, reduced amounts of light available for photosynthesis by plants, and shifts to turbidity-tolerant fish communities.
- All tributaries within the Tahoe Basin deliver sediment and nutrients to a single downstream waterbody: Lake Tahoe. The Tahoe Basin has 63 individual tributaries and associated watersheds, each with its drainage area, slope, geology, and land-use characteristics resulting in high variability throughout the Region. Variability in the amount, timing, and form of precipitation strongly influences runoff patterns. A substantial rain shadow exists across the basin from west to east; precipitation can be twice as high on the west shore relative to the east shore of Lake Tahoe. Both new and legacy disturbances to the landscape can affect the volume of runoff, erosion rates, and the ability of the watershed to retain sediment and nutrients. Landscape disturbances including, but not limited to, impervious road and parking lot surfaces, residential and commercial development, wildfire, and the degradation of stream environment zones, can contribute to sediment and nutrient inputs to the Lake or its tributaries. Weather variations and their effects on stream hydrology (particularly the extremes of droughts and floods), and long-term climate change are considered among the most important environmental drivers of tributary runoff. Domagalski et al., 2021 suggested that the downward trend for both suspended sediment concentrations and loads were likely attributed to forest growth and recovery. Increases in FSP concentrations observed since the big 2017 water year have been attributed to the ongoing impacts of that water year (Smith 2022).
Delivering and Measuring Success
EIP Action Priorities
- Reduce Stormwater Pollution From: Roads and Highways, Forest Roads, Public and Private Parcels
- Restore Priority Meadows, Wetlands, and Lake Tahoe Tributaries
EIP Indicators
Example EIP Projects
- Burke Creek/Rabe Meadow Riparian Restoration Project
-
High Meadows/Cold Creek Restoration
This project completed by the USDA Forest Service restored two miles Cold Creek stream channel in the High Meadows area.
Monitoring Programs
Rationale Details
- Considerably Better Than Target. The 90th percentile concentration for suspended sediment measurements during the assessment period (2020-2023 water years) was 38 milligrams per liter (mg/L) relative to the threshold standard of 60 milligrams per liter (mg/L). The status is considerably better than target.
- Moderate Improvement. The downward trend in suspended sediment concentration was identified Domagalski et al., 2021 suggested that the downward trend for both suspended sediment concentrations and loads were likely attributed to forest growth and recovery. The observed declines were consistent with the findings of an earlier analysis by (Coats, R. 2016). Declining suspended sediment concentrations represent an improvement in conditions relative to the target.
Confidence Details
- High. The USGS lead Lake Tahoe Interagency Monitoring Program (LTIMP) was started in 1979 to help understand why clarity was declining in Lake Tahoe. Currently, LTIMP stream monitoring is a partnership between USGS, U.C. Davis, the Tahoe Regional Planning Agency, Lahontan Regional Water-Quality Control Board, and the California Tahoe Conservancy.
Streamflow and water-quality samples are being collected at pertinent Lake Tahoe tributaries to estimate nutrient and sediment loads to the lake and assess trends in stream water quality. Streamflow and stage are measured at 9 gages in 8 basins within the Lake Tahoe watershed and water quality is monitored at 7 sites in 7 basins. Turbidity is monitored continuously at 5 of the water-quality sites.
Water-quality samples are collected during routine visits, storm events, and snow-melt runoff. Samples collected are analyzed for nitrogen, phosphorus, suspended sediment, and fine sediment at the laboratory, as well as for temperature, specific conductance, pH, dissolved oxygen, and turbidity in the stream.
The standardized stream monitoring protocols and length of record provide high confidence in the data.
Methods for FSP estimation from real-time turbidity measurements have been reviewed by the USGS. - High. A 2021 paper (Domagalski et al., 2021) found that flow-normalized nitrate concentrations trended down at all sites, and that total Kjeldahl nitrogen showed decreasing trends in load and concentration at most sites. The observed declines are consistent with the findings of an earlier analysis of long-term trends in Tahoe tributaries (Coats, R. 2016).
- High. See description of status in confidence.
Additional Figures and Resources
No photos available.
Model Archive Summary for Surrogate Regression of Fine-Sediment
- Uploaded On
- 8/21/2024
- File Type
- Description
- Lake Tahoe tributary total maximum daily load (TMDL)s are specified by FSP (0.5 to 16.0µm) counts per liter. Heyvaert and others (2011 and 2015, p. 41) examined the relationship of turbidity and FSP counts and developed the methods for developing FSP regressions for Lake Tahoe stream regions and concluded additional FSP data were needed from Lake Tahoe Interagency Monitoring Program (LTIMP) sampling locations.