The cobalt blue waters of Lake Tahoe were about as clear in 2021 as they were in 2020. But a broader look at clarity measurements shows there is no pattern of consistent clarity improvement over the past 20 years. The lake also has not fully recovered from a spike of fine particles that flowed into its waters after the extremely wet year of 2017.
That’s according to the by the ÁńÁ«ĘÓƵ, Davis, Tahoe Environmental Research Center. UC Davis has measured clarity and other health indicators at Lake Tahoe since 1968, helping to inform policymakers and stakeholders on strategies to protect the lake and stabilize the decline in clarity that dates back to the region’s development boom in the 1960s. .
Recent years have presented evolving and new threats to Lake Tahoe as climate warming, floods, droughts and wildfires impact the lake in ways that are not fully understood.
“The lake itself is changing internally, and the external inputs that impact clarity and lake health are changing at the same time,” said Geoffrey Schladow, director of the UC Davis Tahoe Environmental Research Center. “We are working with other researchers at Lake Tahoe and with agency partners to not only keep track of clarity, but to adapt management approaches for improving clarity in future years.”
A perplexing pattern
Lake Tahoe’s average annual clarity in 2021 was 61 feet compared to 63 feet in 2020. Summer measurements were 54.8 feet, while winter averages were 71.9 feet.
While clarity in winter months is invariably better than during the summer, the trend from the past two decades indicates that neither summer nor winter clarity levels are improving over time.
Particle problems
Decades of research led to the development of the Lake Tahoe Total Maximum Daily Load, or TMDL, the science-based plan to restore the lake’s historic clarity. TMDL science identified fine particles and tiny algae as playing a large role in determining lake clarity. Currently, these are responsible for up to 70% of clarity loss.
Public and private investments in water quality improvements over the past 25 years have significantly reduced fine particles and algae-feeding nutrients entering Lake Tahoe, and TMDL pollutant load reduction targets are being met.
However, fine particles have remained elevated since 2017, on record at Tahoe. Fine particles in Tahoe’s streams increased by fourfold that year and have remained above the historic mean since that time. Fine