Lake Tahoe's clarity decreased last year, slowing a trend of steady improvement that had seen the cobalt mountain waters reach their clearest level in a decade, researchers said Tuesday.
The decline in clarity - about 9 percent from the previous year's average - is within normal ranges and should not be viewed as a reversal of significant gains made in recent years to protect the lake, scientists said in releasing the annual figures at an environmental symposium.
The decline likely was caused by relatively high precipitation from thunderstorms in 2003, which led to increased runoff of soil and pollutants into the lake, the experts said.
"I view this as neither good nor bad news," said John Reuter, a researcher with the University of California-Davis Tahoe Research Group. He said it reaffirms the need to continue regional planning and environmental programs as well as research to determine the most cost-effective restoration measures.
Visible at depths of 102 feet as recently as 1968, a white plate called a "Secchi disk" could be seen at an average depth of 71 feet last year, the new figures show.
In 2002, it was visible at depths of 78 feet, the clearest in 10 years. The five previous years were:
- 2001, 73.6 feet
- 2000, 67.3 feet
- 1999, 69 feet
- 1998, 66 feet
- 1997, 64 feet
"While the pattern may not be what we want to see or would have predicted, the decrease in Secchi depth is well within the average inter-annual variation in these measurements," said Larry Benoit, manager of the Tahoe Regional Planning Agency's Water Quality Program.
Known for its cobalt blue and azure hues, the 193-square-mile lake that is 1,636 feet at its deepest point contains enough water to cover the state of California to a depth of 14.5 inches. But sedimentation and other pollution are spurring algae growth that threatens to turn Tahoe's waters green.
John Singlaub, the regional planning agency's executive director, said the latest results "demonstrate the continued need for environmental improvements at Lake Tahoe."
"It's important to look at these numbers in the context of the big picture of what's happening and our path for the future," he said.
Research models show the most important factor affecting year-to-year changes in clarity is precipitation, including rain, snowfall and runoff.
Increasing clarity in recent years was mostly due to relatively dry years from 1999 to 2002, Reuter said.