Lake clarity takes dive
Researchers say culprit is precipitation increase
By Gregory Crofton
Tribune staff writer
All the snow that fell last spring and the thunderstorms that struck last summer likely jolted more sediment and nutrients into Lake Tahoe, decreasing its clarity by an average of 7 feet in 2003.
The drop from 78 feet to 71 feet of clarity is a departure from a trend of improvement in lake clarity over the last several years, according to data released Tuesday by the UC Davis Tahoe Research Group.
The improvement likely was due to relatively dry weather at the Lake Tahoe Basin from 1999 to 2002, said John Reuter, a director for the Tahoe Research Group.
“Bottom line, most of the changes we see year to year are most likely the direct result of changes in precipitation,” Reuter said. “A model we’ve done shows that none of the (2003) behavior is fundamentally different than any other year since 1967.”
The Federal Water Master’s office in Reno tracks the amount of precipitation that falls each year at Tahoe City. In 2002, 27.52 inches of precipitation fell; in 2003, 31.73 inches fell.
“I view this as neither good nor bad news, but it reaffirms the critical need for processes such as the Environmental Improvement Program,” Reuter said. “There hasn’t been a fundamental shift in the way things operate up here, which means the level of BMPs still hasn’t reached a level that will have a big impact on the lake.”
Reuter said the best management practices he is talking about include erosion controls, constructing wetlands and increasing the amount of stormwater runoff that infiltrates soil in the basin.
Environmental improvement projects are good, but agencies in the basin need to makes sure that the projects that get constructed offer the greatest available benefit to the environment, said John Singlaub, executive director of the Tahoe Regional Planning Agency, an organization formed in 1969 to protect Lake Tahoe.
“I think we need to really look closely at what specific things we can do that give us the biggest bang for the buck,” Singlaub said. “It’s a very difficult question. As we get into some research that’s coming out of Pathway 2007 we need to look where we can make changes that can better help the lake. We’re still seeing erosion coming out of developed areas in South Lake Tahoe; still seeing an increase in algae growth.”
Data reported by Tahoe Research Group also indicated an increase in algae growth.
“Algae not only scatters light like sediment particles, but it absorbs it too,” said Larry Benoit, TRPA water quality program manager.
The latest research indicates that up to 60 percent of clarity loss can be attributed fine particles of sediment that stay suspended in the lake for long periods of time. The other 40 percent of clarity loss is likely caused by algae.
Rochelle Nason, executive director of the League to Save Lake Tahoe, said the 7 foot decrease in clarity is not a surprise.
“Loss of lake clarity has slowed in recent years but scientists have been telling us to brace ourselves for these losses” Nason said. “This is a wake-up call to redouble our efforts to save Lake Tahoe.”
Lake clarity measurements are taken by the Tahoe Research Group approximately every 10 days. A white disc is lowered into waters just off the West Shore and the depth at which it disappears from site is recorded as the depth of clarity. The readings can be extreme, but it is the average of all the measurements that gets reported once a year.
– Gregory Crofton can be reached at (530) 542-8045 or by e-mail at firstname.lastname@example.org