Tahoe on track to regain clarity
Targets to reduce urban stormwater pollution and help protect Lake Tahoe’s famous water clarity are being achieved by local governments and state highway departments, according to the Total Maximum Daily Load Program’s 2019 Performance Report. The report is compiled each year by the Nevada Division of Environmental Protection and the California Regional Water Board, Lahontan Region.
Launched in 2011, the program is a science-based plan to restore Lake Tahoe’s water clarity back to its historic level of 97.4 feet by 2076. The program requires local governments and highway departments at Lake Tahoe to meet regular targets to reduce clarity-harming pollutants that wash into the lake.
The 2019 TMDL Performance Report shows local governments and highway departments at Lake Tahoe successfully exceeded pollutant reduction goals set for 2018 to decrease “fine sediment loads” (e.g. road sands applied to winterize streets that are subsequently crushed by vehicles and washed into the lake) by approximately 440,000 pounds per year.
Twenty years ago, Lake Tahoe was losing a foot of clarity per year. In 2019, as a result of the Tahoe Basin partnership’s efforts, winter clarity is improving by a half-foot a year. Strategies leveraged to reduce fine sediments and protect Lake Tahoe’s clear blue waters include using environmentally-friendly wintertime road operations practices, technologies and products, and installing stormwater treatment infrastructure that captures and cleans dirty runoff.
“Local governments and highway departments continue to do extraordinary work at Lake Tahoe to help mitigate stormwater pollution and protect the lake’s majestic blue waters,” said Nevada Division of Environmental Protection Administrator Greg Lovato. “Through a collaboration of efforts, we are now over 80% of the way to achieving the 2021 milestone, which is a fine sediment load reduction of 21% from baseline levels.”
Collectively, program activities have directly reduced fine sediment loads by 18 percent from 2004 baseline levels, as well as lowered the levels of phosphorus by 14% and nitrogen by 10.5%.
The TMDL Program’s current strategies focus on fine sediment reduction, as research indicates this pollutant has the greatest impact on clarity and overall lake health. Because developed areas also provide significant opportunities to control fine sediment pollution, restoring Lake Tahoe’s clarity hinges on meeting urban stormwater pollution reduction goals. Stormwater from roads and urban areas contributes to over 70% of fine sediment loads, which scatter light and reduce lake clarity. The nutrients phosphorus and nitrogen can trigger algae growth that also harms lake clarity.
The TMDL is an adaptive management program and the states are committed to continually exploring novel and more efficient ways looking to improve clarity. In 2016, NDEP and the California Tahoe Conservancy funded UC-Davis and UNR to implement a three-year pilot project to remove an invasive shrimp from Emerald Bay. Earlier research had suggested that the shrimp was feeding on native zooplankton and contributing to clarity loss.
Coinciding with a record year for precipitation, snowpack, and summer lake temperature, Lake Tahoe’s water clarity reached an all-time low of 60 feet in 2017. In 2018, annual average clarity rebounded to 71 feet and the five-year running average remains at approximately 70 feet. The trend in clarity for the winter months (December through March) shows a continuous slight improvement since the early 2000s. This is a direct result of the successful implementation of the TMDL and Environmental Improvement Programs, as well as a testament to the overall success of Lake Tahoe basin land management policies, initiatives, and partnerships over the past 40-plus years.
The trend in summer clarity (June through September), however, continues to decline steadily. Nevada and California natural resource agencies brought this issue to the Tahoe Science Advisory Council and asked that they develop a plan leveraging best available science to help inform program management. Priority topics for investigation include the causes of the seasonal trend divergence; a broader assessment of climate change impacts on watershed hydrology, in-lake ecology, and lake clarity processes; and feasibility evaluation of potential management options.
“It is very encouraging that TMDL implementation continues to be on track, though certain key challenges remain. Roadway operations and stormwater treatment facility maintenance costs will become more onerous through time, so securing dedicated funding is critical to ensure ongoing effectiveness of these pollutant controls,” said Lahontan Water Board Executive Officer Patty Kouyoumdjian. “Furthermore, we are starting to see and experience the impact on clarity from a warming climate and lake. Applied science is needed to better understand the magnitude of these impacts and to help guide the TMDL Program to respond and adapt appropriately.”
“The ongoing improvement in winter clarity is proof of what the 75 partners of the Environmental Improvement Program can do when working toward a common goal,” said Joanne S. Marchetta, executive director of the Tahoe Regional Planning Agency. “Summer clarity remains a real concern, but I am confident that when scientists and agencies work together we can continue to rise to the challenge.”