LAKE TAHOE, Calif./Nev. – The Tahoe Regional Planning Agency’s (TRPA) environmental threshold evaluation report, which monitors the regional plan’s environmental factors across 140 indicators, shows that in 2019 to 2023, 75% of the indicators trended positively. The results of the peer-reviewed report can now be accessed online through a dashboard and reveal the positive effects of the Environmental Improvement Program (EIP).

The TRPA prepares a threshold evaluation report every four years—this most recent one represents the 8^th comprehensive threshold evaluation since 1980. The data is collected for a three-year period, then all the data are compiled into a report, peer-reviewed by the Tahoe Science Advisory Council. For the period from 2019 to 2023, 92% of the indicators were stable or improving, while 79% of the thresholds were “in attainment” or within the accepted level.

What are these indicators?

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The primary indicators were decided when the threshold evaluation report was established. But since then, the TRPA updates the thresholds through conversations with different organizations, including state and federal agencies, local nonprofits, and scientists who work in the area.

Each year, the report is published with peer review from the Tahoe Science Advisory Council (TSAC). TSAC was extensively involved in updates for stream environment zones this year, along with the conversations regarding water quality.

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Robert Larsen, program officer at TSAC, told the Tribune that nine experts from across the country were involved in the review process, including an expert on scenic thresholds from Virginia Tech and a scientist from the University of Idaho for soil thresholds.

“It’s important to bring in outside voices and new eyes for peer reviewers, to help get different ideas and approaches on the table,” said Larsen. But no matter where they come from, TSAC associates are aware of the environmental values in the Basin and bring new perspectives to the challenges here.

The measurements collected come from experts in the field, not necessarily TRPA. For example, air quality is measured through partnerships with local air quality districts and the University of California, Davis, which have established air quality monitors in key areas. Other indicators require the use of remote sensing for soil or regular counts of species in the region. Scenic monitoring, one of the last thresholds, is particularly unique as it requires monitoring from roughly 700 different viewpoints throughout the Basin. TRPA helps bring these data together into a single report and dashboard for ease of access.

Holding ground

Dan Segan, chief science officer at TRPA, told the Tribune that the history of environmental degradation in the Basin was what sparked the need for the threshold report. Excessive logging, introduction of invasive species, and land development all threatened the ecosystem and environment.

Now that many of the thresholds are in attainment, it can seem like no progress is being made. “But that’s what victory looks like for these thresholds—holding ground,” said Segan. “That’s indicating that there’s no backsliding on these issues.”

Jeff Cowen, TRPA’s public information officer, attributed the success of the report to the EIP. “Across the board, when we see those successes in attainment or improvement, it’s due to the many projects under the EIP.”

But when there are changes, either in the science or the collected data, then those thresholds may also change.

Unexpected changes

During the collection period, three major changes in thresholds occurred: air quality, aquatic invasive species, and stream environment zones.

The Caldor, Dixie, and Tamarack fires all happened during the data collection period, causing exceedances in air quality thresholds for short periods of time. However, the EIP’s focus on fuel reduction helped control the Caldor Fire, and the TRPA also changed their code to allow mechanical thinning up to 50% on slopes, which extends fuel reduction to areas previously untouched.

In 2023, the New Zealand mudsnail was found in the area—the first new aquatic invasive species detected in the lake in decades. It tipped the aquatic invasive species threshold into the first “out of attainment”, but mobilized many groups to increase the scale and pace of their projects to control them.

The TRPA adopted a new threshold standard to improve stream environment zones, which include meadows surrounding streams—Segan was key in these conversations. Previous targets from 1982 focused solely on acres of restoration, while the new targets also assess the quality of the restoration. While the new threshold is in the improvement zone, TRPA celebrated meeting the previous restoration target within this period, as more than 1,100 acres have been restored.

Transportation and tourism

During the data collection period, the Destination Stewardship Council (DSC) was formed in response to the outdoor recreation issues stemming from the COVID-19 pandemic—they also created the Lake Tahoe Destination Stewardship Plan, which aims to help management of tourism using research tools.

TRPA and the DSC will be working together to adopt new goals for the recreation threshold indicators, including shoreline safety and noise control, as well as corridor management plans that establish transit to popular recreation sites.

This report is the first to address the new threshold category of Transportation and Sustainable Communities, which was adopted in 2021 and addresses transportation, workforce housing, walkability and accessibility, and other community-oriented goals. Funding increases starting in 2022 have raised more than $50 million in new revenues for transportation, bringing nearly $8.5 million to local operating funding for transit in the 2023/2024 fiscal year—the highest ever seen in the Tahoe Basin.

For the 2023 evaluation, the new thresholds of vehicle miles traveled (VMT) and the per capita VMT both declined.

Future questions

Climate change, future fires, and warming weather were some of the factors that TRPA identified as potentially affecting how the thresholds will shake out in the future. And further conversations on how to accurately collect, evaluate, and present the data have been a constant conversation.

One major question in this report was on the ever-elusive lake clarity. Winter clarity has improved, indicating that projects that stop stormwater runoff and sediment collection in the lake are likely working. But summer clarity has been trending down over the years, with no clear answer as to why.

While the TRPA is required to sort the data into discrete categories, Segan and Cowen both emphasized the influence that the factors have on each other. For instance, having less VMT can be related to better transit and walkability in an area, which would then result in more lake clarity and improved air quality.

Segan said, “Policies and projects can help improve things across a number of indicators. Plus, the benefits (of threshold attainment) transcend the categories they’re in. They reflect the bigger picture of environmental protection in Lake Tahoe.”

The report and dashboard can be accessed at https://thresholds.laketahoeinfo.org/. The Draft 2023 Threshold Evaluation will be presented to the TRPA Governing Board during its Wednesday, Feb. 26 meeting.

Eli Ramos is a reporter for Tahoe Daily Tribune. They are part of the 2024–26 cohort of California Local News Fellows through UC Berkeley.