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Home Water Streams and Wetlands

Suspended Sediment Concentration

Status and Trend

Interpretation and Commentary

Status: Somewhat Worse Than Target
Trend: Moderate Improvement
Confidence: Moderate

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Map



The ten streams routinely monitored for suspended sediment include 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 - Sediment (particularly fine sediment) delivered to Lake Tahoe is known to directly affect the transparency of Lake Tahoe (Lahontan and NDEP 2010). The protection and restoration of the Lake’s transparency is a central environmental goal, and Lake transparency is considered a key socioeconomic value. Under the Federal Clean Water Act, and in California under the Porter-Cologne Act, each state develops a set of water quality standards designed to protect the beneficial uses of a waterbody. These standards can be narrative, numeric or both. For California streams in the Lake Tahoe Basin, the standard states that suspended sediment (SS) concentrations in tributaries to Lake Tahoe shall not exceed a 90th percentile value of 60 mg/L (Lahontan 1995). TRPA also has authority to establish standards, and it adopted an equivalent Numerical Standard in Resolution 82-11 (TRPA 1986). Nevada does not have a Numerical Standard for SS, however the TRPA Threshold Standard applies to streams on the Nevada side of the Lake Tahoe Basin. In the Basin, Lahontan Regional Water Quality Control Board (CA), Nevada Division of Environmental Protection (NV), and TRPA have the individual authority to determine if standards have been violated.
  • Adopted Standards  - Attain a 90th percentile value for suspended sediment concentration of 60 mg/L (milligrams per liter).
  • Indicator - The Indicator is suspended sediment concentration (expressed as milligrams per liter; mg/L), but the unit of measure reported here is the proportion of suspended sediment samples that exceeded 60 mg/L each water year (Oct. 1 – Sept. 30). Currently a total of 20-35 individual suspended sediment samples are collected each water year from each of the ten regularly monitored streams. However, the number of individual samples collected each water year has varied over the period of record from 2 to 157. For example, if a total of 30 individual samples were collected from a stream in a given water year, 27 samples (90 percent) would need to have a SS concentration ≤60 mg/L to meet the standard. Conversely, only 3 samples (10 percent) could have a SS concentration > 60 mg/L.
  • Status – In the table below, scores of suspended sediment concentration status, trend, and confidence were assigned for each of the ten regularly monitored streams in the Lake Tahoe Basin. Assigned scores for individual streams for California and Nevada, and overall, were based on: 1) percent to target calculations, 2) standard exceedance rates (see also Appendix WQ-1), 3) visual inspection of graphed data, and 4) the aggregation methods described in the Methodology Chapter of this report. The status for each stream was determined by evaluation of its 2010 value relative to the standard. The trend determination was based on a comparison of the exceedance rate among three periods, 1980-1989, 1990-1999, and 2000-2010. More details on confidence scoring are provided in the confidence section below. Five of the ten monitored streams in water year 2010 exceeded the standard for suspended sediment concentration (see table below and figure above in this indicator summary). The percentage of samples for each stream that exceeded the standard ranged from 11 to 25 percent. Two of the streams were in Nevada (Third and Incline creeks), and three of the streams were in California (General, Blackwood, and Ward Creeks). Ninety percent of all inflow delivered to Lake Tahoe from the ten monitored streams comes from the five California streams; thus, the total contribution of SS from California streams was substantially larger than from the Nevada streams. Due to the relatively larger influence of California streams, the status of tributary SS concentration was determined to be “somewhat worse than the target,” even though the overall average would indicate that the Region was “at or somewhat better than the target.”

    Stream Status Trend Confidence
    Nevada Streams
    Third -2 4 0
    Incline -2 4 0
    Glenbrook 2 2 0
    Logan House 2 2 0
    Edgewood 2 2 0
    Nevada Summary 0.4 1.2 0
    California Streams
    Trout 2 2 0
    Upper Truckee 2 2 0
    General -2 0 0
    Blackwood -2 0 0
    Ward -2 2 0
    California Summary -0.4 1.6 0
    Overall Summary 0.0 1.4 0
    Overall Qualitative Assessment At or Somewhat Better Than Target Moderate Improvement Moderate
       

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  • Trends Aggregation of the individual trends in SS concentrations relative to the standard for the ten individual streams suggest there is “moderate improvement” in overall trend (see table above). The ten monitored streams can be divided into three categories in terms of trend analysis: 1) SS standard infrequently exceeded, and minimally so, 2) SS standard frequently exceeded, but minimally so, and 3) SS standard frequently exceeded by a substantial amount. Streams in the first category include General, Glenbrook, Logan House and Edgewood creeks. With the exception of General Creek, streams in the first category (SS standards infrequently exceeded, and minimally so) have not exceeded the SS concentration standard in any year over the last decade; thus, the streams in this category are determined to exhibit no trend in SS concentration relative to the established Threshold Standard.

    Trout Creek and the Upper Truckee River are examples of streams meeting criteria for the second category: frequently exceeded the standard, but minimally so. During the early to mid-1980s, the proportion of samples per year that was greater than 60 mg/L averaged 20 to 25 percent when the standard was exceeded. For both Trout Creek and the Upper Truckee River, the standard was exceeded more frequently, and by a greater percentage (above the 60 mg/L value) than in the last two decades (1990 – 2010). Since the early 1990s Trout Creek has rarely exceeded the SS standard. Upper Truckee has also seen a reduction in percent of samples with SS concentrations above 60 mg/L. Thus, a trend of moderately improving SS concentration is assigned to these tributaries.

    Streams falling into category number three (i.e., standard frequently exceeded by a large amount) include Ward, Blackwood, Third and Incline creeks. Third and Incline creeks differed from the others in this category in that the number of years SS concentrations exceeded the standard have declined since the early 2000s. A snow avalanche that occurred in the Third Creek watershed in 1986 may explain the increased SS concentrations measured in the late 1980s and early 1990s. In contrast, the routing of Rosewood Creek, a tributary to Third Creek changed in the early 2000s, and Rosewood Creek now enters Third Creek downstream of the established sampling station. This may explain (at least in part) the reduction in yearly average SS concentrations measured in Third Creek over the last decade, and its assignment of a moderately improving trend. The percentage of samples from Blackwood and Ward creeks that are greater than 60 mg/L track each other fairly well, and these watersheds are located next to each other on the West Shore of Lake Tahoe. These watersheds are relatively undeveloped (although Blackwood has historically been highly disturbed), are dominated by more erosive volcanic soils, and receive relatively higher amounts of precipitation compared to the East Shore. It is hypothesized that the higher SS concentrations measured in these streams are driven by local meteorology and runoff characteristics. This is further supported by comparing inflow in combination with estimates of SS loads for Blackwood and Ward creeks (streams with similar flow characteristics). Overall, localized conditions and events are thought to have a strong influence on SS concentrations in the four streams falling into the third category, obscuring any definitive long-term trends.

Confidence
  • Status – There is high confidence in the reliability of the SS concentration data 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 data, field and laboratory, are subject to extensive quality assurance requirements (USGS 2006). Currently, a total of 20-35 individual samples are collected each water year from each of the ten regularly monitored streams. This sampling frequency is considered sufficient to characterize different inflow conditions observed during the water year; however, the sampling frequency has varied over the period of record. 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 sediment, fine particles and nutrients have been developed and customized for use in aquatic systems where concentrations can be extremely low (Goldman et al. 2009). Thus, there is high confidence in the status determination.
  • Trend – Confidence in the trend determinations are all considered “low.” Although there is high confidence in the data, assessments and interpretations of trend are all based on visual inspection of the graphical information (i.e., qualitative).
  • Overall Confidence – Overall confidence in the status and trend determination is “moderate” given the high confidence in status and the low confidence in trend.
  • 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. Some 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). 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
These plots show measurements of suspended sediment (SS) concentration compared to the water quality standard in place for each of ten regularly monitored Lake Tahoe tributaries by water year. The individual bars presented for each water year (Oct. 1 - Sept. 30) represent the proportion of all individual samples collected in a water year that exceeded 60 milligrams per liter (mg/L). A total of 2-157 individual samples were collected, depending on the water year, from each of the 10 monitored streams. The SS standard for both California and Tahoe Regional Planning Agency (TRPA) states that the stream must attain a 90th percentile value for suspended sediment concentration of 60 mg/L. This is interpreted as no more than ten percent of the stream's SS concentration measurements for the water year can exceed 60 mg/L. The horizontal red line represents this standard. The symbol 'x' denotes that sufficient samples were collected and that none of the samples contained SS concentrations greater than 60 mg/L. Years without data or an 'x' means that no samples were collected, or that data is otherwise missing. Stream monitoring data used to evaluate SS concentrations 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

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Last Updated on Tuesday, 13 November 2012 07:10