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Why are aquatic weeds a problem and why focus on the Tahoe Keys lagoons?

faviconThe aquatic weed infestation compromises water quality and degrades other uses within the Tahoe Keys lagoons.  The weed growth impedes boating, is hazardous to swimmers, contributes to sediment loading, releases nutrients that can contribute to harmful algal blooms, and provides sources of continuing infestations of aquatic invasive species in Lake Tahoe.

The shallow, warm waters of the Tahoe Keys lagoons create conditions favorable to the growth of aquatic weeds, and it is estimated that 90% of the available wetted surface in the lagoons has been infested with aquatic weeds. The Tahoe Keys lagoon encompass 172 acres, (including 900 private docks, accounting for much of Lake Tahoe’s boat traffic).  For perspective, all other current lake-wide weed infestations combined account for approximately 20 acres, around 12% of the Keys infestation.

Tahoe Keys Weeds Problem Storymap – Take a Virtual Tour of the Keys to learn more about the weeds problem and what is currently happening to address it.

What causes weed growth in the Tahoe Keys?


The environmental analysis shows that over 90% of nutrients that feed weed and algal growth in the Tahoe Keys come from the existing weeds and sediment layer. Even if all other sources of nutrients were eliminated, the existing weeds will continue to propagate. Learn more from the Tahoe Keys nutrient fact sheet.

What is included in the test project to address the weeds in the Tahoe Keys lagoons? How was it developed?

faviconThe Control Methods Test (approved January 2022 by Lahontan Water Board and TRPA) assesses a variety of aquatic weed control methods, such as Ultraviolet (UVC) light, Laminar Flow Aeration (LFA), aquatic herbicides, and bottom barriers.  This test project will help determine which combination of methods is most effective in controlling aquatic weeds within the Tahoe Keys lagoons and will inform a Long Term Management Plan.  For project details click HERE.

The Control Methods Test was collaboratively developed and written by the lead agencies – the Tahoe Regional Planning Agency (TRPA) and the Lahontan Regional Water Quality Control Board (Lahontan Water Board) – and Tahoe Keys Property Owners Association (TKPOA) with input and guidance from the Tahoe Keys Stakeholder Committee.

Additional Information

In July 2017, Tahoe Keys Property Owners’ Association (TKPOA) submitted an application for an exemption to the Lahontan Basin Plan Pesticide Prohibition, to potentially use herbicide as an aquatic weed control treatment in the Tahoe Keys lagoons. The lead agencies – the Tahoe Regional Planning Agency (TRPA) and the Lahontan Regional Water Quality Control Board (Lahontan Water Board) – performed a preliminary environmental analysis on this proposed project in 2017. The Initial Environmental Checklist for TRPA and an Initial Study under the California Environmental Quality Act (CEQA) for the Lahontan Water Board lacked sufficient data to determine environmental effects, and a full TRPA/CEQA analysis was deemed necessary to evaluate the project.

In July 2018, TKPOA submitted a follow up application for aquatic herbicide use to the lead agencies, expanding the project to a larger scale, called the Tahoe Keys Lagoons Restoration Project.  The project proposed a twelve-year, full-scale treatment plan for target aquatic invasive weeds throughout the entire Tahoe Keys lagoons.  In response, TRPA initiated a collaborative stakeholder process to ensure transparency and close coordination among the lead agencies and key stakeholders in development and evaluation of the project.

Through the collaborative stakeholder process, the full-scale Tahoe Keys Lagoons Restoration Project was modified to the Tahoe Keys Lagoons Integrated Control Methods Test (CMT) project.  Changes were made to include testing of new technologies that may be effective for aquatic invasive weed control.  The CMT is designed to test the efficacy of select aquatic herbicides as well as several non-herbicide control methods to inform what will work best for longer-term management.  The results of the CMT will help inform managers of a long-term, integrated approach that is most suitable for bringing target aquatic weeds under control within the Tahoe Keys lagoons and to better protect Lake Tahoe from their spread.

How does the Control Methods Test relate to a long-term management plan to control the weed problem in the Tahoe Keys lagoons?

faviconResults from the Control Methods Test will inform TKPOA’s long-term Integrated Management Plan (required by Lahontan Water Board) to address aquatic weeds in the Tahoe Keys lagoons.  Control methods approved for testing cannot be implemented on a large-scale until another environmental analysis and public review process is completed.

For more information on the timeline of the project, click here.

What weed control methods are being tested in the Control Methods Test?

faviconDue to the large and complex Tahoe Keys lagoons infestation, The Tahoe Keys Lagoons Aquatic Weed Control Methods Test (CMT) is testing several aquatic weed treatment methods alone and in combination to understand what works in the lagoons unique ecology.  The goal is to determine the best combination of methods for initial large-scale knock-down and subsequent management of target aquatic weed growth.  The CMT includes testing aquatic herbicides, proven small scale control methods (such as bottom barriers and diver assisted suction), along with innovative methods – such as Ultraviolet (UVC) light and Laminar Flow Aeration (LFA) – that have never been used for aquatic weed treatment anywhere other than Lake Tahoe.

Additional Information

Treatment methods for aquatic weed control included in the test project are organized into two groups:

  • Group A – Large-scale treatment methods for addressing target aquatic weeds using aquatic herbicides*, large scale UVC light** and LFA***.
  • Group B – Small-scale, localized treatment methods for addressing target aquatic weeds including UVC light spot treatments, bottom barriers, diver-assisted suction and diver hand pulling techniques. Group B treatments are designed to be used as a follow up treatment to Group A methods.

*Only California and U.S. Environmental Protection Agency (EPA) approved aquatic herbicides are being considered in the analysis.  Three aquatic herbicides have been identified as potential methods of treatment based on the weeds they are intended to target: Endothall, Triclopyr, and ProcellaCOR (ProcellaCOR is approved by the U.S. EPA and awaits approval by California EPA.  It will only be included in tests if California EPA approval is granted).

** Ultraviolet (UVC) light is an emergent technology for treating aquatic weeds. This innovative technology has only been tested on a small scale at Lakeside Marina in an unobstructed environment (no docks or rock walls to maneuver around).  While initial tests in 2018-2019 show promising results, tests in the Tahoe Keys lagoons are needed to understand how effective this technology might be in that specific environment.

*** Laminar Flow Aeration (LFA) is a technique that has been used to improve water quality in water bodies with low levels of oxygen. Anecdotal evidence suggests that LFA can also result in reductions in aquatic weed densities. A pilot project at Ski Run Marina is showing promising results, triggering the inclusion of LFA into the proposed test project. A small scale pilot project in the Tahoe Keys lagoons began in April 2019 and is currently underway (with support from The League to Save Lake Tahoe).

In addition to weed control methods, long-term water circulation and water quality improvement methods are being tested to evaluate how best to control environmental factors favorable to aquatic weed growth.  Methods to control the spread of fragments that break loose from aquatic weeds and start new infestations are also be tested. These methods include LFA to improve water quality (and to reduce plant populations) and Bubble Curtains + Sea Bins for fragment containment and collection.

When will testing of treatments methods begin and how long will it last?

faviconDecision makers approved the Control Methods Test in January 2022, and testing began in spring 2022.  The Control Methods Test includes one to three years of testing each of the approved control methods, with the duration of each test dependent on the control method.  Monitoring of test results is in place during all tests and for two subsequent years to determine efficacy of various control methods.


Have herbicides been used in Lake Tahoe before? Why are they being considered now?

faviconAquatic herbicides have never been permitted for use in Lake Tahoe.  Herbicide use is being tested and analyzed now because existing treatment methods have not brought the rapidly growing infestation under control in the Tahoe Keys lagoons.  Non-chemical, experimental treatments have been and continue to be explored.  Aquatic herbicide may be a part of the solution available to knock back the complex and large-scale infestation before it gets beyond control and further infests Lake Tahoe. The Control Methods Test (CMT) is designed to test various methods side-by side.

Additional Information

Aquatic weed surveys from 2014, 2015, 2016 and 2017 show the extent and density of excessive weed growth in the lagoons (See Figures 2 and 3 in the Project Description).  Of particular concern is the recent rapid growth and spread of curlyleaf pondweed, as current treatment methods have not been effective.  This aquatic weed is difficult to control due to its swift and multiple modes of establishment, along with the large number and dispersal capacity of its asexual turions. Turions are overwintering buds that become detached and spread throughout the waterway and have the potential to remain dormant in the sediment at the bottom of the water for several years, often surviving bottom barrier treatments (Woolf and Madsen 2003, Wittmann et al. 2015, Xie and Yu 2011).

Curlyleaf pondweed is capable of growing in deeper, colder waters, which may be detrimental to Lake Tahoe if allowed to spread unchecked.  The weed’s swift and multiple modes of establishment, coupled with its ability to tether to boats, strongly suggest that this invasive species could become well established in much of Lake Tahoe’s nearshore – depths of 30 feet and less – within the next few years, without the aid of additional control methods.

What is the Antidegradation Analysis included in the environmental analysis (EIS/EIR)? Why is it needed?

faviconAn Antidegradation Analysis is an assessment of the water quality impacts of the proposed control methods, and is a required element for permitting.  Lake Tahoe is designated as a Tier III, Outstanding National Resource Water by the Environmental Protection Agency (EPA) and the state of California. This strict classification makes the bar for permitting the use of aquatic herbicides exceptionally high.  In addition, Lahontan Water Board’s Basin Plan includes a prohibition for the use of aquatic herbicides in Lake Tahoe, and an exemption to this prohibition can only be granted if an Antidegradation Analysis can be passed, and other criteria required by the Basin Plan are met.  The practical importance of this ONRW designation is that no permanent or long-term degradation of water quality is allowed in Tier III waters.  Short term degradation may be approved, generally defined as “weeks to months, not years” of water quality degradation.

The Antidegradation Analysis determined that the use of state and federal approved herbicides, and other proposed methods, can meet the strict environmental standards required for permitting tests.  Lahontan Water Board and TRPA approved the Control Methods Test in January 2022.

For additional regulatory information associated with this project, visit Lahontan Water Board’s website.

Who makes the final decision on what is approved for the Control Methods Test?

faviconThe Tahoe Regional Planning Agency (TRPA) and the Lahontan Regional Water Quality Control Board (Lahontan Water Board) board members considered and approved the Final EIS/EIR and draft permits in January 2022.

What is being done now to protect Lake Tahoe and slow the spread of weeds from the Tahoe Keys lagoons to the rest of the lake?

faviconTahoe Keys Property Owners Association (TKPOA) currently utilizes a variety of best management practices to reduce the volume of weeds present in the Tahoe Keys lagoons and slow the spread of plant fragments into Lake Tahoe.  They include:

Control Methods

Prevention Methods

Small Scale Test Methods

            • Control and mitigation methods that do not require additional permitting on a small scale:
            • Laminar Flow Aeration (LFA)
            • Ultraviolet (UVC) Light

In addition to the work by TKPOA in the Tahoe Keys lagoons to prevent and control aquatic weed infestations, the Lake Tahoe Aquatic Invasive Species Plan guides the collaborative approach to protect the waters of the Lake Tahoe Region from the threat of aquatic invasive species. The plan identifies prevention and education programs such as Boat Inspections and the Eyes on the Lake Program, and encourages strategies that minimize damage from existing infestations.

How is this work being funded?

faviconTahoe Keys Property Owners Association (TKPOA) has invested several million over the last decade to manage the invasive weeds and to develop improved control methods.

Tahoe Regional Planning Agency (TRPA) has secured millions of dollars in funding for the costs of the environmental analysis, an inclusive stakeholder process for the Control Methods Test, testing of UVC light and monitoring of the CMT.  The League to Save Lake Tahoe and TKPOA have partnered to fund the Laminar Flow Aeration test currently underway in the Keys.

The League to Save Lake Tahoe has provided funding to help pay for the Bubble Curtains and Sea Bins that have been installed across the channel of the West Lagoon and are planning on funding the same prevention measures for the channel of the East Lagoon.  These steps buy time, by containing weed fragments in the Keys lagoons, while management solutions are investigated.


What is the project timeline?

faviconAn overall snapshot of the project timeline can be found HERE.

How do I learn more and get involved?

faviconThe most recent project details and reports can be found here  and on the TKPOA website.

If you would like to receive emails with updates from TRPA on this project, please enter your information HERE.  You may submit comments or questions anytime HERE.

Additional project and regulatory information can be found on the Lahontan Water Board website.  If you would like to receive project updates from the Lahontan Water Board, please sign up HERE.