RENO, Nev. - Reeling from decades of development, Lake Tahoe's famous cobalt blue hue will turn a murky green within 30 years without intense restoration efforts, a new study says.
''Lake Tahoe is gravely imperiled,'' government scientists and university researchers concluded in the report being released Wednesday.
''The biological integrity of many aquatic ecosystems in the basin appears to be at risk,'' according to a copy of the executive summary obtained by The Associated Press.
The $2.6 million study was launched two years ago as a result of President Clinton's environmental summit at the Sierra lake on the Nevada-California border.
It concludes that several animal species once abundant in the Tahoe basin are vanishing or have been lost, including the Lahontan cutthroat trout, Sierra Nevada red fox, willow flycatcher and yellow-legged frog made famous by Mark Twain's story ''The Jumping Frogs of Calaveras County.''
''The scientific evidence suggests that recent human impacts are largely responsible for the current decline in the clarity of the lake,'' the report concludes.
''Progress made to date, unfortunately, does not diminish the immediacy and urgency of the problems needing redress,'' according to the researchers at the University of Nevada, Reno, and the University of California, Davis.
Scientists are trying to figure out how humans affect the lake, from the logging of the late 1800s, to the housing development boom around the lake in the 1960s and air pollution in neighboring urban centers.
''With perhaps as few as 10 years left to halt or reverse the loss of clarity in the lake, the basin's entire natural and social system must be considered together,'' the report said.
Tahoe's clarity continues to decline at a rate of about one foot a year as a result of the 5 percent annual growth in algae, the 1,200-page study says.
The algae growth has been spurred primarily by atmospheric deposits of nitrogen as well as shoreside erosion and water runoff with phosphorus-rich sediment.
Everything from fertilizer on golf courses and smoke from fireplaces to automobile emissions and home building is contributing to the problem.
A white dinner plate used to measure Tahoe's clarity was visible at depths of 105 feet in the 1960s.
But that same plate is visible only as deep as 66 feet today and would be visible only as deep as 40 feet by the year 2030 if the decline goes unchecked, the report said.
''The ability of Lake Tahoe to dilute nutrient and sediment inputs to levels where they have no significant effect on lake water quality has been lost,'' said John Reuter, associate research ecologist at UC Davis and director of the Lake Tahoe Interagency Monitoring Program.
The report says efforts in Congress to secure hundreds of millions of dollars to finance additional research and protection measures are critical to reversing the staggering decline in lake clarity.
''Current levels of funding for research and monitoring in the areas of best management practices effectiveness, source identification and control and treatment of runoff in the Tahoe basin is inadequate to meet the demands,'' the report said.
The researchers concluded about half the nitrogen and one-fourth of the phosphorous in the lake is deposited there from the atmosphere. They have been unable to confirm the specific sources, but note that nitrogen compounds are largely associated with automobile emissions and phosphorous compounds appear to be tied to wood smoke and road dust.
The assessment plays down fears that the Tahoe Basin could be the site of a massive wildfire. It says Tahoe's forests are unnaturally thick and subject to insect attack partly as a result of clear-cutting by loggers more than a century ago and the fire suppression practices that followed.
The prehistoric Tahoe watershed was at times a rather smoky place in which fires burned at low intensity during summer and fall, said Tom Cahill and Steve Cliff, UC Davis researchers who contributed to the report.
''The recent conditions are an aberration, achieved by putting out naturally occurring fires,'' Cahill said. ''Every square foot of the forest of the basin used to burn every 30 years.''
Dennis Murhpy, a University of Nevada researcher, told the Reno Gazette-Journal that all the researchers agree there's no time to waste.
''If we don't start making measurable headway, we're going to lose our ability to fix the lake in the next decade.''