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Home Water Streams and Wetlands

Suspended Sediment Load

Status and Trend

Interpretation and Commentary

Status: No Target Established
Trend: Litle or No Change
Confidence: Moderate

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Map


The ten streams routinely monitored for sediment load includes five streams in Nevada and five streams in California: ninety percent of the cumulative total inflow from the monitored streams is from the five California streams, and ten percent is from the five Nevada streams. The sub-watersheds where stream monitoring occurs are colored in the figure above.
  • Relevance - This indicator measures how much suspended sediment is delivered to Lake Tahoe via ten regularly monitored streams (measured as suspended sediment load). Sediment (particularly fine sediment) delivered to Lake Tahoe is known to directly affect the transparency of Lake Tahoe (Swift et al. 2006). The protection and restoration of the Lake’s transparency is a central environmental goal, and Lake transparency is considered a key socioeconomic value. The tributaries have been identified as one of four source categories of pollutants (i.e., sediment and nutrient) loading to the Lake (Lahontan and NDEP 2010). LTIMP stream monitoring data are used to evaluate the status and trend of tributary loading of sediment and nutrients to Lake Tahoe. Long-term stream monitoring is also important to detect changes in water quality that may occur as a result of watershed restoration, or as a result of uncontrollable drivers such as weather and climate change.
  • Adopted Standards  - 1)Tributaries: reduce total yearly nutrient and suspended sediment load to achieve loading thresholds for littoral and pelagic Lake Tahoe; 2) Littoral and Pelagic Lake Tahoe: decrease sediment load as required to attain turbidity values not to exceed three NTU(Nephelometric Turbidity Units). In addition, turbidity shall not exceed one NTU in shallow waters of the Lake not directly influenced by stream discharges (load reduction needed to attain standard, not provided).
  • Indicator - The yearly load for each stream is calculated as the sum of daily loads for a given water year. Currently, a total of 20-35 individual samples are collected each water year from each of the ten streams. The combined yearly load represents an estimate of the total mass of suspended sediment transported by ten streams to Lake Tahoe during a single water year. Indicators measured include suspended sediment load (expressed as metric tonnes/year, metric tons/year, or MT/yr[1]) and fine sediment particle load (expressed as number of particles/yr). Fine sediment particles are less than 16 microns in diameter.

    [1]1 Metric ton = 2,205 pounds
  • Status – The combined yearly suspended sediment (SS) load for water year (WY) 2010 was 7,649 MT. The combined yearly load of fine sediment particles for WY 2010 was 2.45 x 1019 particles. The total yearly inflow for all ten streams was 176,000,000 (or 1,760 x 105) cubic meters in WY 2010. SS loads were largely driven by annual loads from Blackwood and Ward creeks, and the Upper Truckee River. Together they contributed 76 percent of the average load from all ten streams (Appendix WQ-2). Fine sediment particle loads were largely driven by the yearly loads from the Upper Truckee River, and Blackwood and Trout creeks, which together contributed 78 percent of the average yearly load from all ten streams (Appendix WQ-2). There is no clearly established Numerical Standard (target) for suspended sediment or fine sediment particle loads, resulting in a status determination of “unknown.”


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  • Trends Tributary sediment loads are a function of concentration and amount of inflow (which is a function of annual precipitation). Combined yearly tributary loads of suspended sediment and fine sediment particles are strongly influenced by inflow, and in the Lake Tahoe Basin there can be considerable year-to-year variation in combined total yearly inflow from the ten monitored streams (Appendix WQ-2). Tributary inflow depends on the amount and timing of precipitation (snow and rain) that falls in each watershed. Factors such as air temperature and snowmelt, rain-on-snow events, and rain versus snow, all affect the timing of inflow within the water year. Subsurface infiltration and stormwater discharge from developed areas also affects tributary flows; thus, estimates of sediment load presented here represent an unknown proportion of non-urban and urban sources. There are circumstances where yearly load does not relate well to yearly stream flow. This is usually associated with water years that have a large rain-on-snow event (e.g., 1997 and 2006). These events can be intense and erosive, generating more material than other runoff events such as spring snowmelt runoff.

    From water years 1993 to 2010, the total yearly inflow ranged from 772 to 3990 x 105 cubic meters. During the 18-year period, total yearly inflow was highest in water years 1995 through 1998, and 2006, and was above 3000 x 105 cubic meters in those water years. Inflow was extremely high in water year 1997 when a huge rain-on-snow event caused massive flooding, especially on the West Shore. These types of large hydrologic events have the capacity to transport significant amounts of material to the Lake as they are intensive and erosive. For the period 1999-2010, total yearly inflow was moderate to low, ranging from 827 to 2770 x 105 cubic meters, except for the large total yearly inflow in water year 2006––the result of a large New Year’s rain-on-snow event. Water Years 1994, 2001, 2002, 2004, and 2007 to 2009, were characterized by low total yearly inflow (less than 1500 x 105 cubic meters) from the ten monitored streams.

    The combined yearly SSloads for the ten monitored streams during water years 1993-2010 ranged from a low of 1,000-2,000 Metric Tons in 1994, 2001, 2002, 2004, and 2007, to greater than 50,000 MT in 1997. Generally, there is a strong positive relationship between total yearly inflow and combined yearly suspended sediment load for the ten streams, with the notable exception of water year 1997, when the ratio of suspended sediment load to flow was higher than in other years. Flow-weighted concentration in 1997 also increased by a factor of two or more relative to the other water years, except 2006. The values measured in 1997 are thought to be a direct result of increased erosion during the extreme 1997 rain-on-snow event.

    The evaluation of trend in sediment loads from tributaries to Lake Tahoe is complicated by the large inter-annual variability in precipitation and inflow and the response of individual streams. It would be erroneous to make conclusions based on apparent short-term trends in combined yearly loads, given the known variability in yearly inflow. Yearly flow-weighted concentrations (FWC) of sediment were calculated to investigate long-term pollutant loading trends in the presence of variable inflows (Appendix WQ-3). Further, a five-year moving average of sediment FWC was constructed to help reveal long-term trends. While the five-year moving average of sediment FWC appears to decline after the early 2000s, this is more a function of the moving average being influenced by the large 1997 inflow peak and the associated, elevated FWC. Thus, visual inspection of the five-year moving average suggests no trend in FWC of suspended sediment, and this observation was used to establish a trend of “little or no change” in suspended sediment load.

    The load of fine sediment particles less than 16 microns in diameter has only been measured since water year 2002. Yearly combined fine sediment particle loads for the ten streams during water years 2002 - 2010 ranged from 0.473 x 1019 particles in 2008 to 9.88 x 1019 particles in 2006. Combined yearly fine sediment particle loads generally follow a similar pattern as SS loads. However, the ratio of SS load to fine sediment particle load for any particular year appears to depend on the general magnitude of flow, except for water years 2006 and 2008, which had the highest and lowest total yearly inflows, respectively, in the 2002-2010 period. Overall, fine sediment particle loads exhibit no clear trend either in the aggregate or in the individual streams (Appendix WQ-2).

Confidence
  • Status – There is high confidence in the reliability of the data used to calculate yearly loads as the data collection consistently followed national field monitoring protocols established by the U.S. Geological Survey for stream monitoring (USGS variously dated; Rowe et al. 2002). All field and laboratory data are subject to extensive quality assurance requirements (USGS 2006). The stream monitoring program focuses on both event-based conditions (large runoff events associated with rainfall and snowmelt) and baseline conditions (low inflow during summer when precipitation is negligible). The analytical methods for measuring fine sediment particles has been developed and customized for use in aquatic systems where concentrations can be extremely low (Goldman et al. 2009).
  • Trend – There is high confidence in the reliability of the data used to calculate yearly flow-weighted concentrations as the data collection followed national and consistent field monitoring protocols established by the U.S. Geological Survey for stream monitoring (USGS variously dated; Rowe et al. 2002). However, a visual inspections of the five-year moving average was used to estimate the long-term trend in suspended sediment load, yielding a “low” confidence in trend assessment.
  • Overall – Because there is high confidence in status and low confidence in trend, the overall confidence was determined to be “moderate.”
  • Human and Environmental Drivers - Within the Tahoe Basin, all the tributaries deliver sediment and nutrients to a single downstream water body, Lake Tahoe. Lake Tahoe has 63 individual tributaries and associated watersheds, each with their own drainage area, slope, geology, and land-use characteristics. Furthermore, variability in the amount, timing, and type of precipitation strongly influences runoff patterns. A substantial rain shadow exists across the lake from west to east, where precipitation can be twice as high on the West Shore relative to the East Shore. Both new and legacy disturbances to the landscape can affect the volume of runoff, erosion rates, and the ability of the watershed to retain nutrients. Landscape disturbances including, but not exclusive 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 long-term climate change are considered among the most important environmental drivers of tributary runoff.
  • Monitoring Approach – The LTIMP stream monitoring program was first developed in 1979 to provide a Basin-wide evaluation of sediment and nutrient input from tributaries to Lake Tahoe, and to support research efforts that aim to understand the drivers affecting the transparency of Lake Tahoe. Ten streams have been monitored since the early 1990s; five in California (Upper Truckee River, and Trout, General, Blackwood and Ward Creeks) and five in Nevada (Third, Incline, Glenbrook, Logan House, and Edgewood creeks). Six of these streams have been monitored since water years 1980 or 1981. A few of the ten streams have had multiple monitoring stations along the tributary and all have primary monitoring stations at or near the point of stream discharge to Lake Tahoe. Currently, a total of 20-35 individual samples are collected each water year from each of the monitoring stations. The tenprimary stations allow for the evaluation of the cumulative conditions within the watershed and represent approximately 50 percent of the yearly tributary inflow into Lake Tahoe (Lahontan and NDEP 2010). Reporting of combined yearly loads begins in water year 1993 because that was when the 10th stream was included in the monitoring program. U.S. Geological Survey gauging stations are located at each of the monitoring stations, where inflow (discharge) measurements are collected and continuous inflow is calculated. Other water quality-related constituents monitored include water and air temperature, pH, specific conductance, and dissolved oxygen.
  • Monitoring Partners – U.S. Geological Survey (Nevada and California Water Science Centers), University of California at Davis (Tahoe Environmental Research Center), Tahoe Regional Planning Agency, and U.S. Forest Service – Lake Tahoe Basin Management Unit.

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Trend Charts

Chart data not available at this time
Combined yearly sediment loads (metric tons/year of suspended sediment with total yearly inflow for tenstreams routinely monitored in the Lake Tahoe Basin). Data are displayed for each water year (Oct. 1 - Sept. 30) from 1993 through 2010. The yearly load for each stream is calculated as the sum of daily loads for a given water year. The combined yearly load represents an estimate of the total mass of suspended sediment transported by ten streams to Lake Tahoe during a single water year. The dashed line is the combined total yearly inflow from the ten streams, which represents approximately 50 percent of the inflow from all 63 tributaries to Lake Tahoe (Lahontan and NDEP 2010). Stream monitoring data used to calculate loads are from the sampling locations closest to where the tributaries discharge to Lake Tahoe. Data are from the Lake Tahoe Interagency Monitoring Program (LTIMP).

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References

Additional Information

Last Updated on Tuesday, 13 November 2012 07:08