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Miles of Stream Habitat in Marginal Condition

This indicator has been combined with the other stream condition indicators and is reported at https://thresholds.laketahoeinfo.org/ThresholdIndicator/Detail/173

 

This indicator measures the amount of marginal stream habitat in streams of the Tahoe Basin. Stream habitat includes features such as gravel, boulders, large wood, pools, riffles, and riparian vegetation that provide habitat for a wide variety of organisms. Streams are critical to the Lake Tahoe Basin’s water cycle by feeding freshwater to lakes and ponds, recharging groundwater, providing habitat for a wide variety of aquatic and terrestrial organisms and corridors for fish and wildlife migration. Past practices such as gravel mining, logging, and develpoment along stream corridors negatively impacted fish habitat. Regional programs such as removing impassable culverts and stream habitat restoration projects aim to improve stream habitat. Physical stream habitat (large woody debris, boulders, etc.) and benthic macroinvertebrates (mayflies, stoneflies, etc.) are used to monitor the health of Tahoe's streams.

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Status

Average California Stream Condition Index (CSCI) scores of all "trend" sites for each time period. 

Evaluation Map

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"Status" monitoring locations and streams in the Tahoe Basin rated as marginal, good, and excellent from 2009-2014.

2015 Evaluation

Status
Considerably Worse Than Target
Trend
Insufficient Data to Determine Trend
Confidence
Low
View Evaluation

Applicable Standard

Maintain the 38 miles of marginal stream habitat as indicated by the Stream Habitat Quality Overlay map, amended May 1997, based upon the re-rated stream scores set forth in Appendix C-1 of the 1996 Evaluation.

Key Points

  • There are more stream miles in marginal condition than the goal for the threshold standard. 
  • There is a slight decrease in the amount of stream miles in marginal condition due to several large restoration projects and better stream flows during the 2016-2019 period compared to the drought years of 2012-2015.

About the Threshold

Streams and their associated riparian habitats are key components of the Tahoe Basin’s aquatic ecosystems and are important to people. Concern for stream water quality and biological condition is embodied in various federal, state, and regional water quality laws, regulations, and ordinances, including the Clean Water Act, 208 Water Quality Plan, TRPA Code of Ordinances, and California/Nevada state water quality standards. Streams and associated environments significantly contribute to the Tahoe Basin’s biological diversity and provide recreational opportunities for people in the Basin. The use of benthic macroinvertebrates and associated physical/chemical stream measurements (i.e. bioassessment) to assess overall biotic health of streams is a widely accepted practice used by the EPA and all 50 states in their water quality programs.
A suite of natural environmental factors including weather and climate patterns, especially drought, and geological context such as geological origin, elevation, topography, and soils influence stream condition and habitat suitability for a variety of fish species. Past resource extraction has contributed to legacy effects on the physical features of streams and their biota. Channel modifications associated with historic logging activities (e.g., dams, water extraction and diversions, flumes, stream channelization, and flood control impoundments) altered stream channel structure and watershed-specific hydrology. Historic grazing damaged stream banks and soils and altered stream channel habitat structure through sedimentation and the simplification of riparian plant structure and composition. The impact of these activities can be seen in the high percentage of sand and fine sediments from excess erosion in many of the Tahoe Basin streams. However, the unique geological features found in the Tahoe Basin can also increase the amount of natural sand and fines found in these streams. Dams can create barriers to movement and migration of aquatic organisms and alter natural stream flow patterns. Several factors within developed areas contribute to the alterations of key stream features including: 1) the urban transportation infrastructure, 2) land cover and disturbance, 3) urban landscaping practices, and 4) water withdrawal and export. Roads can contribute sediment and chemical inputs, thereby altering streambed conditions and elevating chemical pollutant loads. Road crossings can confine streams from natural meander patterns, resulting in impediments to organism movements, stream bank instability, and channel downgrading. Increased impervious surfaces on the landscape can prevent water from naturally percolating into soils thereby affecting its rate of delivery to streams. As a result, organisms downstream of developed areas can experience more intense flooding events and flashier flow regimes as the water moves faster from the land into the channel. However, in an analysis of Tahoe Basin stream health in relation to impervious cover of the watershed, little correlation was found. The study found that less than three percent of all monitoring sites had watersheds with impervious cover greater than five percent. Numerous studies have shown that impacts to streams generally begin to appear when five percent to 10 percent impervious cover is reached, meaning that the majority of Tahoe Basin streams do not exceed this impervious cover threshold. However, once there is a more robust stream sample size, it will be worthwhile to reassess the impacts of impervious cover on Tahoe stream health. Forest structure and fires also impact stream flow and water quality. The forests of the Sierra Nevada today are denser today as result of fire suppression than they were 200 years ago. The result of denser stands is likely reduced stream flow and increased forest thinning could increase annual average streamflow by as much as 6%. Fires can also dramatically alter surface dynamics and sediment and nutrient yields from burned areas. The Angora fire in 2007, which burned 22% of the watershed area, had a significant but not catastrophic impact on water quality in Angora creek.

Delivering and Measuring Success

EIP Indicators

Example EIP Projects

Monitoring Programs

Rationale Details

Results from stream sampling spread out across the entire Basin (n=92) between 2009-2014, indicate that 55 percent of streams are in excellent condition (considerably better than the target of 34 percent), The proportion of the streams in excellent condition well exceeds the target and is encouraging news for stream health in the Basin. The main concern is the proportion of streams in marginal condition. Being below the target for proportion of good streams is less worrisome as most of these streams instead are in the excellent category. The high number of marginal sites is being addressed through stream restoration and stormwater management, among other activities. Low water levels caused by drought are likely the largest contributor to poor biological health of at least four of the 24 marginal sites (4.3 percent of all streams included in the sample). Consequently, if sampling only occurred during normal or above normal water years, the proportion of marginal streams would likely be lower and closer to attainment.

In addition to BMI sampling, physical stream habitat data is collected. Physical habitat is a good indicator of the stream’s ability to provide habitat for fish, BMIs, and other aquatic life (Kaufmann et al. 1999). California’s Surface Water Ambient Monitoring Program used stream data compiled across California’s Sierra Nevada and North Coast regions to assess the linkages between habitat quality and biotic integrity of a stream. Out of all physical stream attributes, the following were deemed to have the closest link to biotic health: percent sand/fines of the substrate, level of human disturbance in the riparian area, intactness of woody riparian cover, and overall fish cover (large woody debris, undercut banks, overhanging vegetation, boulders, etc.) (Rehn 2015). Based on the data, break points were identified where, if below, the streams were very likely (greater than 90 percent) to have poor biological condition (Rehn 2015). Here is a summary of how Tahoe Basin streams sampled by TRPA measured against these attributes:
• Percent sand/fines of substrate: 63 percent of streams were above the break point
• Riparian disturbance: 72 percent of streams were above the break point
• Woody riparian cover: 81 percent of streams were above the break point
• Overall fish cover: 89 percent of streams were above the break point
Water temperature also plays a large role in stream health in the Tahoe Basin. Temperatures above 22 degrees Celsius are widely regarded in the literature as an acute stress threshold for salmonid species above which metabolism is impaired, fitness declines, and mortality increases (Purdy, Fesenmyer, and Henery 2014). In a 2012 study, continuous data loggers found water temperatures in the Upper Truckee River from Christmas Valley to Lake Tahoe were found to exceed 22 degrees Celsius for over 300 hours over the summer (Purdy, Fesenmyer, and Henery 2014). These high stream temperatures, in addition to high rates of stream bank erosion and high percent sand/fines of the streambed, likely play a large role in the low CSCI scores observed in the Upper Truckee River. Exposure to elevated temperatures below 22c can result in chronic stress in Salmonids. (Wenger et al. 2011; Isaak et al. 2012; Luce et al. 2014). Climate change is likely to both shift peak flows and increase stream temperatures in the Region, both of which influence the suitability of the Region’s streams for Salmonids(Jager, Van Winkle, and Holcomb 1999).
Insufficient data to determine trend. There are 48 trend sites and each site is sampled every other year (24 each year). For this analysis, there are three monitoring periods. Each monitoring period consists of two years of data collection (beginning in 2009/2010) since half of trend sites are sampled in one year and the other half the following year. The average score for trend sites decreased slightly since monitoring began, with an average CSCI score of 0.96 in 2009/2010 to an average score of 0.93 in 2013/2014 (Tahoe Regional Planning Agency 2015b). However, because only three monitoring periods exist it is determined to be insufficient data to determine trend. Additionally, no statistically significant trends in physical habitat were found during the monitoring period (Tahoe Regional Planning Agency 2015a).

The impact of low flow on stream health is well documented (Mazzacano and Hoffman 2007), and recent drought conditions are likely impacting BMIs and overall stream health. Streams that rely more on snowmelt than groundwater and springs appear to be showing the greatest effects. Four trend and reference sites that went dry (Glen Alpine, Cascade, Ward, and General creeks) during the drought years of 2013 and 2014 saw their average CSCI score drop from 0.925 during the above average water years of 2010 and 2011 to an average score of 0.716 during the drought years of 2013 and 2014, a decrease of 23 percent (Tahoe Regional Planning Agency 2015b). Because human impacts around and upstream of these sites are minimal and are not likely to have occurred during the time period, this drop in biotic integrity is believed to be the result of natural factors. The change in these four sites accounted for 78 percent of decline in average site score. If these sites maintained their above average water year scores during times of drought the overall trend for all sites would be a decrease of 0.3 percent for this monitoring period, as opposed to the decrease of 1.42 percent that was observed. If the drought persists, it is likely the overall trend in stream health will continue to decline.

Confidence Details

High. There is high confidence in the status. A large number of sites (92) covering the basin are sampled following well established and published protocols for assessing stream biotic integrity.
Low. Because there are only three monitoring periods represented, confidence in the trend is low.
Low. Overall confidence takes the lower of the two confidence determinations.

Additional Figures and Resources

No documents available.