Indicator
Suspended Sediment Load (Tributaries)
2015 Evaluation
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2015 Evaluation
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Status
Insufficient Data to Determine Status or No Target Established
Trend
Moderate Improvement
Confidence
Low
Evaluation Map
Suspended Sediment Load Map.JPG
Description

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.

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.
Outcomes
Programs and Actions Implemented to Improve Conditions
Stream environment zone (SEZ) restoration and enhancement, urban growth control limits, best management practices (BMPs) to reduce nutrient and sediment discharge from disturbed soils, retrofit regulations for private and commercial property BMPs, reducing private automobile use through improvements to public transit and alternative transportation modes (with the goal of reducing air pollution and the subsequent deposition of nitrogen and fine sediment), and ongoing allocation of water quality mitigation funds to support erosion control and storm water pollution control projects. Projects completed by EIP partners since between 2009 and 2015 have:
• Restored or enhanced 27,150 linear feet of stream channel
• Retrofitted 120.55 miles of road and decommissioned an additional 7.4 miles of road
• Restored or enhanced 120 acres of disturbed forested uplands
• Inspected 108.72 miles of unpaved non-urban roads and maintained 98.2 miles
• Issued 18,076 BMP certificates to commercial, multifamily and single family residential properties
Effectiveness of Programs and Actions
Quantitative evaluation of the effectiveness of any individual policy, program or action implemented to improve the tributary water quality is challenging because of the diversity of contributing factors. Each of the programs and actions are thought to aid in improving (or preserving) tributary water quality; however, the signal from any of these individual actions cannot be discerned from the year-to-year variability in suspended sediment loads monitored as part of the LTIMP Tributary Monitoring Program. Although there is high inter-annual variability associated with suspended sediment yield and load, and both are strongly related to stream flow, the trend analysis finds a significant long-term decline in suspended sediment load overall. This suggests the adverse effects of legacy development and watershed disturbance have subsided, while urban growth limits, implementation of water quality BMPs, more protective forest management practices, and watershed restoration are having a positive influence.
Interim Target
Because this is a management standard with no defined numerical target, one cannot reasonably establish an interim target.
Target Attainment Date
Because this is a management standard with no defined numerical target, one cannot reasonably predict when this threshold standard will be attained.
Recommendations
Analytic Approach
The load calculations used in these indicator sheets were derived from procedures developed in an applied science project for the USDA Forest Service, with funding from the Sierra Nevada Public Land Management Act (Coats and Lewis, 2014b, 2014a)(Coats and Lewis, 2014 a,b). Software compatible with the updated water quality data base also were provided. It is recommended that the revised methods for calculating total loads continue in use for now, but that they be revised and updated as improvements (such as use of turbidity as a covariate, and development of new statistical models for total load) become available. With each updating, annual loads should be recalculated for the entire period of record using the improved method.
Monitoring Approach
Suspended sediment loads and yields should continue to be monitored to track long-term trends; however, coordinated additional work such as focused studies and effectiveness monitoring, are needed to assess the causes of those changes. This includes assessing the effectiveness of watershed restoration projects, and understanding the effects of uncontrollable drivers such as weather and climate change. Only five of the seven streams have continuous turbidity monitoring, and a priority should be made to add continuous turbidity in Third and Incline creeks. In addition to increasing the confidence of annual suspended sediment load estimates, this monitoring will be useful as an explanatory variable in future estimates of suspended sediment, total phosphorous, and possibly fine sediment particle numbers. Continuous turbidity monitoring, which the USGS initiated in water year 2015, will provide confidence limits at the 90 percent level of +/- 20 percent of the estimated load with only 20 discrete water samples per year (27 samples are currently required provide that level of confidence).
Modification of the Threshold Standard or Indicator
The load reduction needed to attain any of the three standards is not provided, which precludes objective evaluation of standard attainment. Consideration should be given to the establishment of specific standards that enable objective determination of status. Standard review should consider consistency with the TMDL program and include input from federal, state and local agencies, and the science community
Attain or Maintain Threshold
No changes recommended. Continue to pursue the strategies and actions identified in the Lake Tahoe TMDL and Regional Plan with a goal of reducing tributary loading of sediment and nutrients, and achieving the interim target for Lake Transparency by 2031.
Additional Figures and Resources

No documents available.