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Tahoe Clarity Improves to Five-Year High

By Sylvia Wright on April 24, 2002 in Environment

Editor's Note: A chart of clarity measurements since 1968 is available; contact Sylvia Wright, below.

Lake Tahoe was the clearest in 2001 that it has been in five years, said University of California, Davis, researchers today. The Tahoe Research Group, which has studied the famous alpine lake and recommended recovery strategies for more than 40 years, said the finding is promising but does not mean that the battle is won.

"Based on the historical data, lake clarity would need to improve for at least another five years to declare that things are doing very well," said John Reuter, a Tahoe Research Group ecologist and director of the Lake Tahoe Interagency Monitoring Program. "We're extremely encouraged that the clarity in the past few years is better. But we have seen temporary improvements before."

Led by Charles Goldman, director and founder of the Tahoe Research Group, UC Davis researchers have warned that, if unchecked, the long-term decline in transparency will turn Tahoe's famous cobalt-blue waters to green.

The researchers measure lake clarity every seven to 10 days by lowering a white Secchi disk into the water at fixed locations and noting the depth at which the disk disappears.

The annual average Secchi measurements for the past six years were:

o 2001: 73.6 feet (22.44 meters)

o 2000: 67.3 feet (20.53 meters)

o 1999: 69 feet (21.04 meters)

o 1998: 66 feet (20.14 meters)

o 1997: 64 feet (19.51 meters)

o 1996: 76.2 feet (23.45 meters)

In 1968, when UC Davis clarity studies began, the disk could be seen at a depth of 102.4 feet (31.22 meters).

The Tahoe Regional Planning Agency (TRPA) is the bi-state agency leading the collaborative effort to achieve and maintain established environmental quality standards in the Tahoe Basin. TRPA adopted the Lake Tahoe Environmental Improvement Program in 1997 and since then has facilitated restoration efforts in the basin.

Many of those efforts aim to reduce the runoff of sediment, which makes the water cloudy, and of chemicals and minerals that promote algae growth, such as fertilizers.

Reuter said the basin-restoration activities probably contributed to the improvement seen in 2001. However, 2001 was a drought year, which also reduced runoff. In contrast, the worst recent year for lake clarity, 1997, was a flood year.

"Since short-term changes in Lake Tahoe's quality are strongly affected by precipitation and pollutant load in runoff, the long-term impacts of restoration must be teased out from temporary conditions of drought and flood. Complex natural processes take time to discern," Reuter said.

"However, we can say without hesitation that if we don't continue to follow TRPA's Environmental Improvement Program, and even expand it, the chances of seeing clarity improve are much reduced. As a matter of fact, they are very unlikely."

Responding to the new clarity report, TRPA executive director Juan Palma said, "While it is tempting to think that the region's investments in soil erosion control and stormwater treatment are starting to pay off, we know we must approach this data with caution.

"The clarity readings should motivate us to do more to implement the EIP and do it faster. Residents and visitors alike must remain committed to meeting this challenge."

The UC Davis Tahoe Research Group works on scientific problems in the basin in collaboration with local, state and federal agencies, the private sector, the University of Nevada, Reno, and the Desert Research Institute. Its water-clarity monitoring program is largely funded by grants from TRPA, the state of California's Lahontan Regional Water Quality Control Board and others.

UC Davis is planning to begin construction soon on a new $13.2 million laboratory and education facility, the Lake Tahoe Center for Environmental Research and Education.

"Our decades of studies suggest that once nutrients reach the lake, their effect persists for about 50 years. We can hardly hope for an immediate, statistically significant improvement," said UC Davis' Goldman. "In the meantime we need to help the public understand the problems and the best solutions that science and management can provide for long-term restoration."

Media contact(s)

Kat Kerlin, Research news (emphasis on environmental sciences), 530-752-7704, 530-750-9195, kekerlin@ucdavis.edu

Charles Goldman, Tahoe Research Group, (530) 752-1557, crgoldman@ucdavis.edu

John Reuter, Tahoe Research Group, (530) 304-1473, jereuter@ucdavis.edu

Juan Palma, Tahoe Regional Planning Agency, (775) 588-4547

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