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Watercraft Shoreline Test

Status and Trend

Interpretation and Commentary

Watercraft Shoreline Test

Status: Somewhat Worse Than Target
Trend: Insufficient Data to Determine Trend
Confidence: Low

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  • Relevance - This indicator measures noise generated from motorized watercraft in the shorezone of Lake Tahoe. Residents and visitors are concerned about the declining serenity of their communities and their enjoyment of the outdoors due to excessive noise from sources such as motorized watercraft (TRPA 2007c). Excessive recreational boat noise can also disturb sensitive wildlife. For this reason, TRPA adopted policies and ordinances to reduce noise levels and maintain serenity in Lake Tahoe’s shoreline area, while continuing to afford recreational opportunities.
  • Adopted Standards  - Zero single-event exceedances of the Shoreline Test noise limit (>75 dBA) (according to past interpretations of this standard).
  • Indicator - Number of exceedances and exceedance rate (exceedances/day/year) of the Single Noise Event, Watercraft Shoreline Test Threshold Standard measured in A-weighted Decibel (dBA).
  • Status – TRPA’s maximum allowable noise level from any motorized watercraft in the shorezone, regardless of distance to shore, is 75 decibels (dBA). According to the past interpretations of this standard, 1 exceedance of this standard places the Region in “non-attainment” with the adopted Threshold Standard. Measurement of noise levels in the shorezone from 2009 to 2011 documented exceedances of the motorized watercraft shoreline test Threshold Standard in each monitoring year and therefore, according to previous interpretation of Threshold Standard compliance, the Region is in non-attainment with the zero exceedance Threshold Standard. Monitoring efforts documented that motorized watercraft were responsible for 47 of 301 (16%) recorded shorezone exceedances in 2009 (0.12 exceedances/day, 1 exceedance every 8.2 days, n=386), 51 of 446 (11%) recorded exceedances in 2010 (0.11 exceedances/day; 1 exceedance every 8.7 days, n=446), and 91 of 458 (20%) recorded exceedances in 2011 (0.21 exceedances/day; 1 exceedance every 4.7 days, n=428)(TRPA 2011c). When placed in context of the total number of boat trips undertaken over the same time period, data from boating surveys conducted in 2009, 2010, and 2011, indicate that 203,821, 188,047 and 193,540 boat trips occurred in those years, respectively. Within that context, TRPA recorded noise exceedances from boats in 0.0002% to 0.0005% of the trips undertaken, or a greater than 99.999% Threshold Standard compliance rate. Although the Region is not achieving the zero exceedance Threshold Standard, when viewed in context of the number of boat trips recorded over the same time period, it suggests that the Region is “somewhat worse than target.”

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  • Trend Although a simple linear regression model estimated that the exceedance rate is increasing at a rate of 0.045 exceedance/day/year, the trend in the noise standard exceedance rate was given the category “insufficient data to determine trend” based on the limited monitoring record (2009 to 2011). The relationship between exceedance rate and time was not statistically significant (R2=0.681, P=0.391), and likely the result of small sample size (n=3 years).


  • Status – There is a “high” degree of confidence in status because of noise monitoring protocol(SAE 1970),unit accuracy, data integrity, and spatial representativeness of monitoring sites.
  • Trend – According to methods for determining confidence in trend for this evaluation, the trend was determined to be “moderate” in exceedance rate because the probability of the trend occurring by chance was less than 0.4 and greater than 0.1 (i.e., there is 60.9% certainty that the trend is occurring). However, due to the small sample size (only 3 years sampled) and inter-annual variability in exceedance rate, confidence in trend is more appropriately classified as “low” with nearly a 40% probability that there is no trend (or little or no change).
  • Overall Confidence - The overall confidence in the status and trend was determined to be “low” because there was a “low” confidence determination for trend and “high” confidence determination for status. More monitoring efforts are needed to improve confidence.
  • Human and Environmental Drivers - Watercraft-generated single-event noise exceedances are driven by the type of watercraft engine and exhaust system(Lanpheer 2000) and boater behavior (proximity to shore, operating speed, etc.). Shoreline topography and wind can potentially also influence noise levels.
  • Monitoring Approach – Watercraft-generated noise levels are measured annually at ten shorezone monitoring locations for five to six sampling periods (ranging from 4 to 12 days) from May through September. Sampling periods are comprised of both weekend and weekdays, allowing for analysis of the differences in noise levels or exceedances between days in the week. The monitoring periods include low, medium, and high watercraft use times throughout the day (7a.m.-7p.m.). All noise events are individually analyzed and categorized by a trained noise technician.
  • Monitoring Partners – Monitoring was conducted by TRPA with land access granted by the California Tahoe Conservancy, U.S. Forest Service, California Department of Parks and Recreation and Nevada Division of State Parks.


  • Monitoring Plan
  • Conceptual Model


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Locations where UC Davis measures primary productivity and other water quality parameters.

Trend Charts

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The average number of exceedances/day for motorized watercraft determined by the shoreline test (>75 dBA) from 2009 to 2011. According to available data (n=3 years) the exceedance rate on average is increasing at a rate of about 0.045 exceedances/day/year. However, confidence in trend was determined to be low due to low sample size and variation in inter-annual exceedance rate. Source: TRPA shorezone monitoring data. TRPA Threshold Indicator Standard (0 exceedances)

Additional Info


Additional Information

Last Updated on Friday, 26 October 2012 15:24