Tahoe clarity worsening over summers

As scorching heat waves bombard the Western U.S. and Lake Tahoe breaks records of its own, climate change impacts are being felt throughout the watershed.

The Tahoe Regional Planning Agency and UC Davis Tahoe Environmental Research Center released a report of Tahoe’s annual clarity measurements in which the effects of escalating temperatures were front and center. Lake clarity, a key indicator of the health of the watershed, averaged 62.9 feet in 2020, and while that is within the range of the last 10 years, the pattern of clarity readings shows a troubling divergence in seasonal trends. Clarity is holding steady in colder months, but summer results are worsening.

A concurrent set of recommendations from the Tahoe Science Advisory Council underscores the importance of continuing existing policies and programs that strike at the main drivers of clarity change — fine sediments and nutrients that fuel algae growth. Stormwater treatment and stream restoration programs have brought about significant reductions in those pollutants and TRPA will continue to strengthen partnerships in the region to see those programs through.

Researchers continue to increase our understanding of climate change and the advisory council is also working to identify key modeling and monitoring needs to more fully account for the warming climate’s effects on lake clarity and ecological processes.

Tahoe’s warming is also tightening the grip that aquatic invasive weeds have on some of Tahoe’s shallow areas. Aquatic invasive weeds, namely Eurasian watermilfoil and curlyleaf pondweed, have established in some areas of the South Shore and have infested nearly all 172 acres of lagoons inside the Tahoe Keys. These weeds spread easily and flourish in warmer waters. As temperatures rise, more of the lake becomes suitable habitat. Managing this threat is a top priority and TRPA has brought together a broad spectrum of partners including property owners, nonprofits, researchers, and federal and state agencies. Under the Lake Tahoe Environmental Improvement Program, together we are implementing the science-based Aquatic Invasive Species Control Action Agenda to bring AIS under control.

Since 2017, 62 acres of lake bottom have been treated for aquatic invasive species and a team of divers is using proven techniques to knock back a 100-acre infestation of weeds outside of the Tahoe Keys. This work is dangerous because it is in a high-traffic area. Boaters need to stay clear of working boats and markers indicating divers under water. Conflicts with motorized boats, paddlers, and swimmers can also be avoided by respecting the 600-foot no wake zone around the lake. Innovative methods are being expanded, including use of a specially equipped vessel with large UV-light panels.

Still, the Tahoe Keys remains ground zero for aquatic invasive weeds. Property owners there have invested millions trying to control AIS and have applied for permits to test a mix of control methods to finally turn the tide. TRPA and the Lahontan Regional Water Quality Control Board are analyzing the proposal, which includes targeted testing of federally approved aquatic herbicides to establish safe, long-term control strategies for AIS that were introduced to Lake Tahoe decades ago.

The intense heat of this summer is also challenging watercraft inspectors working to keep new aquatic invasive species out of the Tahoe Region. As more people turn to boating to escape the heat, watercraft inspectors are intercepting an alarming number of vessels with invasive species onboard. By early July of this year, 21 vessels carrying an invasive quagga or zebra mussels have been decontaminated by boat inspectors, rivaling previous years and only halfway through the season.

Managed by TRPA and the Tahoe Resource Conservation District, the Lake Tahoe watercraft inspection program is the gold standard for invasive species prevention and has become a model for programs across the nation. Since 2008, there have been no new invasive species detected in the waters of the region. We’re grateful for boaters’ commitment to protecting the lake in this way. By knowing their vessel’s history and following the steps to Clean, Drain and Dry their craft, boaters are helping stop the spread of AIS. It is imperative that we keep up the good work inspectors are doing every day to protect our waters for all to enjoy.

Protecting and restoring Lake Tahoe as we confront rising temperatures is challenging, but there are established programs and policies at work that will bolster the resiliency of the region. As with every solution for Tahoe, epic collaboration and strong partnerships are the foundation on which we build.

Joanne S. Marchetta is the executive director of the Tahoe Regional Planning Agency.