[Water_news] 1. DWR'S CALIFORNIA WATER NEWS - Top Item for 5/13/08

Department of Water Resources

California Water News

A daily compilation for DWR personnel of significant news articles and comment

 

May 13, 2008

 

1.  Top Item -

 

 

 

Hopes rise for keeping Tahoe blue as clouding trend slows

Sacramento Bee – 5/13/08

By Chris Bowman, staff writer

 

Scientists who for decades reported that famously clear Lake Tahoe was turning murkier have discovered that its clarity actually has been stabilizing since 2001.

Using a new, more sophisticated statistical analysis of environmental data, researchers also determined that a reduced rate of visibility loss in the lake was likely the payoff from decades of erosion control, purchases of environmentally sensitive land and restrictive building rules designed to curb runoff.

 

"It's a good hypothesis that the land use restrictions and erosion controls have something to do with it," said John Reuter, a lake scientist with the University of California, Davis, Tahoe Environmental Research Center.

The findings, released Monday, mark the most encouraging development in 40 years of monitoring the clouding of Lake Tahoe, according to Charles Goldman, a UCD professor who in the 1960s was the first to foresee Tahoe's troubles, and then act on its behalf.

 

"There's promise in this data that we've crossed the line," Goldman said.

 

"That's excellent news," cheered Rochelle Nason, executive director of the League to Save Lake Tahoe, known for its "Keep Tahoe Blue" bumper stickers.

"We have good reason to believe the measures to protect Lake Tahoe are indeed improving the clarity, and this news supports that," Nason said.

 

The findings also are striking in light of recent evidence that global climate change has increased air and lake temperatures in the basin, according to Geoffrey Schladow, director of the Tahoe research center. That warming could increase snowmelt and polluted runoff into the lake.

 

"The new data … and other models, suggest that the lake's clarity can be improved even in the face of gradually warming temperatures," Schladow said.

California, Nevada and the federal government have poured millions of dollars annually into preserving 193-square-mile alpine lake – "a noble sheet of blue … not merely transparent, but dazzlingly, brilliantly so," as Mark Twain saw it 136 years ago.

 

Tahoe still dazzles. But erosion, construction runoff and air pollution have reduced its clarity by nearly one-third since the 1968, an average loss of a foot a year.

UCD scientists measure clarity every 10 to 14 days from a university research vessel. They lower a dinner plate-sized "Secchi disk" into the lake at two locations and note the depth at which the white instrument disappears from sight.

 

In 2007, the disk was visible at an average depth of 70.2 feet, a 2.5-foot increase in clarity from the previous year.

Goldman said he expects visibility will be even better this year.

 

"We've had some incredible transparency measurements so far this year," Goldman said, noting that at one location the disk could be seen at a record depth of 148 feet.

 

Lake clarity fluctuates with the weather. Visibility dips in colder years with more rain and snow, which leads to more runoff of sediment and fertilizer. Phosphorus and nitrogen fuel algal growth, which clouds the lake. Tiny suspended particles from vehicle exhaust and road dust scatter light in the water, further diminishing visibility.

Using a computer model that Schladow developed, researchers filtered out the influence of precipitation on lake clarity, to see whether the 2001-2007 leveling of transparency held. It did, confirming that the lake's visibility was on a new trajectory.

 

"We have had seven years in which clarity has consistently been better than the long-term trend would have predicted," Schladow said. "This is unprecedented."

Tahoe researchers stopped short of calling the new trend an "improvement" because visibility has diminished over time.

 

Seven years is too short a span to determine if the new pattern will hold, accelerate or revert to the old, murky trend, scientists said. But it's long enough to raise hopes of a bluer Tahoe – and encourage designation of more public money for its preservation, said John Singlaub, executive director of the Tahoe Regional Planning Agency, which regulates construction in the 500-square-mile basin.

 

Federal, state and local agencies together have spent more than $500 million to reduce runoff and improve water quality through Tahoe Environmental Improvement Program, launched by President Clinton in 1997, according to the Tahoe planning agency. About $213 million of California's share went to build roadside basins that capture runoff from lakeside highways, a major source of lake pollution.

 

"I have to think that the investments we have made to improve water quality and that residents have been making with best management practices on their properties are having a payoff," Singlaub said.#

http://www.sacbee.com/101/story/933092.html