Lake Tahoe's cobalt blue hue is easily seen while parasailing over the water. The lake's latest clarity measurement is within the range it has been for the past eight years, UC Davis researchers said.
Latest data on Lake Tahoe's clarity make it crystal clear that a controversial muddying of the waters by development has slowed or ceased.
UC Davis scientists said last week that the alpine lake was clear to an average depth of 69.6 feet in 2008, despite a prolonged fallout of ash into the lake from fires in nearby Sierra Nevada forests.
Though a half-foot decline from the previous year, the latest clarity measurement is within the range it has been for the past eight years, said researchers who have monitored the lake since 1968.
Environmental regulators and advocates say the reduced rate of visibility loss is likely a payoff from decades of erosion control, conservation purchases, wetlands restoration and some of the world's most restrictive building rules.
Just over $1 billion has been invested in environmental improvements over the past 10 years by local, federal and state officials from California and Nevada, and developers, according to the Tahoe Regional Planning Agency, which regulates development in the 500-square-mile Lake Tahoe Basin.
"We feel we may have turned the corner in the effort to restore lake clarity to levels seen a generation ago," said Joanne Marchetta, the agency's newly appointed executive director.
Restoration efforts, however, must accelerate to offset recently documented effects of climate change, UC Davis researchers said. They suspect global warming could limit the lake's natural mixing cycle, triggering disruption of the food web and robbing it of a clear, cobalt blue hue that helped make Tahoe a world-class tourist destination.
Historically, clouding has been the result of urban pollution originating mainly within the basin, many studies indicate: Sediment enters the lake through stream and urban runoff. Fine particles stay suspended in the water for years, scattering light and lowering visibility. And local road dust, vehicle exhaust and wood smoke end up in the lake.
The particles include nitrogen and phosphorous – the same chemicals in fertilizer – that fuel growth of light-absorbing algae, further reducing transparency.
UC Davis scientists measure visibility every 10 to 14 days from a university research boat. They lower a dinner-plate sized "Secchi disk" into the lake at two locations and note depths at which it vanishes from sight.
Clarity measurements in 2008 ranged from a maximum depth of 122.2 feet on April 24 to a minimum of only 36.9 feet on Aug. 5, according to John Reuter, an expert in water quality and lake ecology and associate director of the UC Davis Tahoe Environmental Research Center at Incline Village, Nev.
Last summer scientists recorded the shallowest clarity readings in 40 years of monitoring – an average 50.5 feet – possibly because of heavy smoke from wildfires elsewhere that drifted into the High Sierra basin and hung around for weeks, Reuter said.
Ash particles could have blocked light penetration into the water and fed the lake's algae, he said.
By comparison, Tahoe's own Angora fire, which in 2007 destroyed about 250 homes and scorched 3,100 acres on the south shore, had a "negligible impact" on lake clarity and algal growth, according to the research center's latest state of the lake report.
The lake's visibility has dropped an average 1 foot a year, from about 100 feet in the 1960s to between 65 and 70 feet today.
In that time, the Tahoe basin's year-round population swelled from about 10,000 to about 60,000.
Last year, UC Davis researchers published a ground-breaking computer analysis showing that since 2001 the lake's clarity actually has been leveling.
