Indicator
Phosphorus 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
Moderate
Evaluation Map
Nitrogen Tributaries Map.JPG
Description

The seven streams routinely monitored for total phosphorous load includes 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 date to determine status. There is no clearly established numerical target for total phosphorus load for any of the standards identified above; thus, no determination of status can be determined. The combined average total phosphorus load (1981 to 2014) for the seven monitored streams was 9,709 kilograms per year, ranging from 1,823 (2001) to 32,983 (1982) kilograms per year, with a median value of 6,660 kilograms per year. Individually, the seven monitored tributaries (1981 to 2014) varied considerably in their total phosphorus yield per unit area and loads (see Table 1). Blackwood Creek had the highest yield per unit area, although the Upper Truckee River, with the largest watershed area, contributed a greater total phosphorus load to Lake Tahoe. The steep and highly-developed watershed of Third Creek had a disproportionately high total phosphorus yield, but the small size of its contributing area resulted in a low average total phosphorus load. The Trout Creek basin was the smallest contributor of total phosphorus yield per unit area, but its large watershed area made it the third largest contributor of total phosphorus load to Lake Tahoe.
Trend – Because the inter-annual variability in total phosphorus 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. The annual total phosphorus load for each of the seven streams was regressed against the associated total and maximum annual daily discharge, and tested for trends in the residuals. The downward trends were significant (P< 0.0035) for Blackwood, General, Third, and Ward creeks, and the Upper Truckee River. However, all of the streams except Blackwood Creek showed a leveling off or even an upturn in total phosphorus load beginning about water year 2000.

The basin-wide trend in total phosphorus load was analyzed by regressing the total annual total phosphorus load (summed over the seven stations) against the sum of the annual total and annual maximum daily discharge. The downward trend shown in Figure 1 is significant at P< 0.00025. As with the trends for four of the streams, the total trend shows a leveling off after about water year 2000, due to some high positive residual values after 2005, and low values between 2001 and 2004.

Percent changes in regression-estimated total phosphorus loads for given annual hydrologic conditions are presented in table 2. For example, at a given total annual and maximum annual discharge in Ward Creek, the estimated total annual total phosphorus load in 2014 was (on average) 42.4 percent less than it would have been for the same hydrologic conditions (had they occurred) in 1973. Trends are based on day-time samples only. Streams without a significant trend are not shown.

The reason for the 20-year downward trend and subsequent leveling off is unclear. It is hypothesized that the 20-year downward trend is due to long-term recovery from the 19th century clear-cut logging and mid-20th century land development. The record-low residuals of 2001 to 2004 may be an effect of the flushing of channel sediment (Simon et al., 2003). The cause of the high positive residual values after 2005 is unknown.
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 total phosphorus load depends on the number of samples and the variance of the daily loads that are sampled to derive the annual load. Coats and Lewis (Coats and Lewis, 2014b, 2014a) presented tables that can be used to estimate the confidence intervals for average conditions in Tahoe Basin streams, for total phosphorus. Currently the LTIMP collects about 20 to 30 discrete water samples per year at each station. With the method used here to estimate total phosphorus loads based on 25 samples per year, we can be 90 percent sure that the true load is within +/- 20 percent of the estimated load
Moderate. The downward trend shown in Figure 1 is significant at P< 0.00025. However, the total trend shows a leveling off after about water year 2000, due to some high positive residual values after 2005, and low values between 2001 and 2004.
Low. Overall confidence takes the lower of the two confidence determinations.
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 to 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. High inter-annual variability in loads, which is thought to be primarily driven by variability in annual precipitation, complicates the determination of overall effectiveness of the TRPA Regional Plan and actions taken by Regional partners. Although there is high inter-annual variability associated with total phosphorus loads, and loads are strongly related to stream flow, the trend analysis finds a significant long-term decline in total phosphorus loads. 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 targets, one cannot reasonably establish an interim target.
Target Attainment Date
Because this is a management standard with no defined numerical targets, one cannot reasonably establish an interim target.
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). 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 become available. With each updating, annual loads should be recalculated for the entire period of record using the improved method.
Monitoring Approach
Total phosphorous loads 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 total phosphorus load estimates, this monitoring will be useful as an explanatory variable in future regression estimates of suspended sediment, total phosphorus, and possibly fine sediment particle numbers. With the method used here to estimate total phosphorus loads based on 25 samples per year, we can be 90 percent sure that the true load is within +/- 20 percent of the estimated load. The use of continuous turbidity monitoring would improve the precision of the estimates. With turbidity as an additional predictor variable, 17 samples per year would provide the same level of confidence that now requires 25 samples per year.
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. Objective determination of “attainment” status for standards without a specific target is a recurrent challenge both in the Region and in the larger field of monitoring and evaluation (M&E). The standard should be assessed against best practice for the establishment of standards and indicators for M&E, and amended as necessary to improve the evaluability of the standard and the information it provides for management. Standard review should consider natural variation in yearly stream flow, consistency with the TMDL program, and include input from federal, state and local agencies, and the science community.
Attain or Maintain Threshold
Continue to pursue the strategies and actions identified in the Lake Tahoe TMDL, Regional Plan and Regional Transportation 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.