University of California, Davis researchers announced Tuesday that Lake Tahoe's clarity remained steady in 2005, averaging 72.4 feet, a foot below the 2004 average of 73.6 feet.
Scientists who work closely with Lake Tahoe said the latest number is encouraging, but indicated the average for this year - not expected until next summer - will be a real test of how well Lake Tahoe can stand up to pollutants.
The 2006 average will reveal the impact of one of the rainiest winters in recent memory, a season that filled the lake this summer by more than 5 feet.
Massive rainstorms rocked the region in late December, leaving frequent pot holes and unusually large erosion gullies along roads and trails.
"The current year will be a real test of how the basin is responding (to environmental improvements)," said Geoff Schladow, UC Davis researcher.
Schladow measures the lake's clarity every seven to 10 days to produce a yearly average announced the next summer. The average announced Tuesday is for data collected during 2005.
The 2005 average keeps clarity in the range of where it's been for the past five years, and where it was for a few years in the 1990s. Lake clarity dipped to 64 feet in the late 1990s, following a record-setting winter flood in 1997.
Scientists learn the most from extreme events, Schladow said.
"We may learn a lot this next year, after this very wet winter, there was a lot of rain, a lot of snow, and it was happening pretty much into June," Schladow said.
Lauri Kemper, a division director at Lahontan water board, was cautious about how this year will turn out.
"We are concerned that the clarity for this year is going to be less than the one for last year, primarily because of the heavy winter that began in December but continued for many months into this year," she said.
Lahontan and other regulatory agencies have aimed for 100 feet of clarity, the average that came out during studies of the lake during the late '60s and '70s.
Lahontan and UC Davis announced last month that a computer model has shown that goal is achievable if all pollutants entering Lake Tahoe are reduced by 35 percent. The primary culprits are fine sediment and nutrients that feed algae.
It is still possible to go out on Lake Tahoe and see 100 feet down, Schladow said, because the clarity number is an average.
"It's never that one value, it's always higher or lower," he said. He said he has seen clarity vary between 135 feet and 46 feet in the same spot on the lake.
Clarity
numbers
2005: 72.4 feet
2004: 73.6 feet
2003: 71 feet
2002: 78 feet
2001: 73.6 feet
2000: 67.3 feet
What pollutes Lake Tahoe?
Scientists believe sediment and nutrients play equal roles:
• Fine sediment from soil erosion and dust scatters light and clouds the water
• Nutrients from fertilizers, air pollution and soil erosion feed algae.