Suspended Sediment Load (Tributaries)
Suspended Sediment Load (Tributaries) is include in the Threshold Dashboard. Threshold Indicators are evaluated against Threshold Standards every 4 years. Thresholds are environmental goals and standards for the Lake Tahoe Basin that indirectly define the capacity of the Region to accommodate additional land development.
Status
Combined yearly load (million metric tonnes/year) of suspended sediment with total yearly inflow (million cubic meters) for the seven streams currently monitored in the Lake Tahoe Basin. Data are displayed for each water year (October 1 to September 30) from 1982 through 2014. The combined yearly load represents an estimate of the total mass of suspended sediment is transported by seven streams to Lake Tahoe during a single water year. The solid line is the combined total yearly inflow volume from the same streams, having a combined contributing watershed area of 350.51 square kilometers. Data is from the Lake Tahoe Interagency Monitoring Program (LTIMP).
Evaluation Map
The seven streams routinely monitored for suspended sediment load include two streams in Nevada: (1) Third Creek, and (2) Incline Creek; and five streams in California: (3) Trout Creek, (4) Upper Truckee River, (5) General Creek, (6) Blackwood Creek, and (7) Ward Creek.
2015 Evaluation
- Status
- Insufficient Data to Determine Status or No Target Established
- Trend
- Moderate Improvement
- Confidence
- Low
Applicable Standard
Reduce total annual nutrient and suspended sediment load to achieve loading thresholds for littoral and pelagic Lake Tahoe.
Key Points
No Key Points
About the Threshold
- This indicator measures how much suspended sediment is delivered to Lake Tahoe via seven regularly monitored streams (reported as suspended sediment load). Suspended sediment, especially fine suspended 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 to Lake Tahoe have been identified as one of four source categories of pollutant (i.e., sediment and nutrient) loading to the Lake (Lahontan and NDEP, 2010a).
- All the tributaries within the Tahoe Basin deliver sediment and nutrients to a single downstream water body: Lake Tahoe. The Tahoe Basin has 63 individual tributaries and associated watersheds, each with their own drainage area, slope, geology, and land-use characteristics resulting in high variability throughout the Region. Furthermore, variability in the amount, timing, and type 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 its 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.
Delivering and Measuring Success
Transportation Indicators
- Acres of Environmentally Sensitive Land Acquired
- Acres of Forest Fuels Reduction Treatment
- Acres of Habitat Protected
- Acres of Habitat Restored or Enhanced
- Acres of SEZ Restored or Enhanced
- Disturbed Areas Restored, Enhanced, or Created
- Fine Sediment Load Reduction
- Forest Roads Created
- Forest Roads Inspected and Maintained
- Impervious Coverage Retired
- Linear Feet of Stream Channel Restored or Enhanced
- Miles of Roads Decommissioned or Retrofitted
- Miles of Street Sweeping
- Nitrogen Load Reduction
- Non-Compliant Wood Stoves Removed or Retrofitted
- Parcels Inspected for Defensible Space
- Parcels with Stormwater BMPs
- Phosphorous Load Reduction
- Tons of Biomass Utilized
Rationale Details
- Insufficient data to determine status. There is no clearly established numerical target for total suspended sediment load for any of the standards identified above; thus, no determination of status can be determined. Annual suspended sediment load is strongly influenced by the volume of total annual runoff. Over the period of complete record (1989 to 2014) the total annual suspended sediment load for the seven monitoring stations averaged 7,520 metric tonnes/year, with a median value of 4,320 tonnes/year, and a range of 0.790 tonnes/year (in 2001) to 53,200 tonnes/year (in 1997). Although water year 1997 had the highest total annual suspended sediment load in the record, it was only the sixth highest year for total annual runoff. A flood in January 1997 had an estimated return period of more than 100 years (Rowe et al., 1988), and contributed to the high sediment loads in that year. In the analysis of time trends, it was found that the annual maximum daily discharge as well at the total annual discharge explains a significant fraction of the variance in total annual suspended sediment load.
Blackwood Creek continues to be the largest source of suspended sediment entering the lake in terms of both yield and load, due most likely to the predominance of highly erodible volcanic tuff in its headwaters, and a history of land disturbance (logging, overgrazing and gravel mining) in the 19th and early 20th centuries. The Upper Truckee River has a relatively low suspended sediment yield per square kilometers of contributing watershed area, but its large size makes it the second-greatest contributor of suspended sediment load. The steep and highly-developed watershed of Third Creek contributes disproportionately to the total load, while Trout Creek is a relatively small contributor despite its large area, due perhaps to the relatively gentle slope of the extensive flood plain in the lower part of the watershed. - Because the inter-annual variability in suspended sediment load is driven largely by the variability in annual runoff, it is virtually impossible to recognize long-term trends in load without first removing the effect of hydrology. After regressing total annual suspended sediment load at each station with total annual discharge and maximum annual daily discharge, the residuals for each station were plotted against water year, and the trends tested using the Mann-Kendall trend test. The results showed highly significant downward trends in suspended sediment load for Incline Creek (P< 0.0008), Third Creek (P< 0.0001), and the Upper Truckee River (P< 0.0049). When the combined suspended sediment loads, total annual discharge, and maximum annual daily discharge for all seven watersheds are considered together, the downward trend is highly significant (P< 2.1 x 10-5). The smoothed trend line is shown below.
Table 2 shows the percent changes in regression-estimated suspended sediment loads for given annual hydrologic conditions. For example, at a given total annual and maximum annual discharge in Third Creek, the estimated total annual suspended sediment load in 2014 was (on average) 68.1 percent less than it would have been for the same hydrologic conditions (had they occurred) in 1983.
Third Creek suspended sediment load showed the strongest downward trend. Simon et al. (2003) also found this trend based on a downward shift in the sediment rating curve following the flood in January 1997. They attributed the downward trend to the flood-induced flushing of sediment stored in the channel, and to recovery of the watershed following heavy development. Glancy (1987) showed that development during the early 1970's in the watersheds of First, Second, Third, Wood and Incline creeks resulted in an average 10-fold increase in sediment load (Glancy, 1987). hose impacts are apparently now declining as the watersheds heal.
Several analytical steps were taken to examine the basin-wide significance of the identified downward trends in suspended sediment load, after exclude the possible leveraging effect of Third Creek: (1) the sediment loads, total annual discharge, and maximum annual daily discharge for the other six stations were pooled; (2) the effect of hydrology was removed; and (3) a trend in the residuals was tested. The results showed that even without Third Creek, the downward trend in suspended sediment load (1982-2014) is still highly significant. The trend can most likely be attributed to long-term watershed recovery from disturbance of land development. There also may be an effect from the flushing of stored channel sediment by the flood of January 1997(Simon et al., 2003). Such an effect could persist for several years, and would not have been removed from the annual residuals by regression.
Confidence Details
- Low. Where insufficient data exists to determine status, confidence in the status determination is low. The confidence in an estimate of total annual suspended sediment load depends on the number of samples, and on the variance of the daily loads that are sampled to derive the annual load. (Coats and Lewis, 2014b, 2014a) presented tables that can be used to estimate the confidence intervals for average conditions in Tahoe Basin streams, for suspended sediment, and for several forms of nitrogen and phosphorous nutrients. In the past, LTIMP stream sampling collected about 20 to 35 discrete water samples per year at each station. With the method used here to estimate total suspended sediment suspended sediment loads based on 27 samples per year, we can be 90 percent sure that the true load is within +/- 30 percent of the estimated load. For the early 1980s, when more than 100 samples per year were collected at each station, the 95 percent confidence limits are +/- 10 percent of the estimated load.
- High. When the combined suspended sediment loads, total annual discharge, and maximum annual daily discharge for all seven watersheds are considered together, the downward trend is highly significant (P< 2.1 x 10-5).
- Moderate. Overall confidence takes the middle of the two confidence determinations when high and low.
Additional Figures and Resources
No documents available.