Cores of sediment taken from the floor of Lake Tahoe indicate it would take 10 to 20 years to restore the lake's fading clarity, according to a UC Davis research scientist.
But that time frame is only valid if nothing bad, like dirty stormwater and crushed road sand, gets into the lake. For decades now a lot of bad things have ended up in Tahoe and caused it to lose 30 feet of its famed clarity.
Today four times as much sediment erodes into the lake each year than was the case before the basin started to be developed around 1860, according to a doctoral dissertation completed in 1998 by Alan Heyvaert, a member of the UC Davis Tahoe Research Group.
"What we have now is more of a chronic disturbance," Heyvaert said. "It isn't going to go away ... but we are trying to mitigate it with Best Management Practices."
Heyvaert determined lake clarity can be renewed in a relatively short period of time by studying sediment layers from the Comstock era, a period of intense logging in the Lake Tahoe Basin that started around 1860 and lasted about 40 years.
Most of the basin was clear cut because of a need for wood to fuel a silver mining boom in nearby Virginia City. By 1900, the basin's barren landscape no longer kept soil containing things like nitrogen and sediment from getting into the lake.
"It was a pulse disturbance," Heyvaert said. "We can't say definitively that the lake clouded. We can say the sedimentation rate in the lake increased about 10 times over pre-disturbance levels ... and make the assumption that sedimentation affected lake clarity."
During the Comstock era, the amount of sediment discharged into the Tahoe was seven to 12 times pre-disturbance levels. Recovery began around 1900 and it was swift. Data collected from sediment layers show the lake recovered between 1910 and 1920, Heyvaert said.
The 2-to 3-foot long sediment cores came from as deep as 1,650 feet in the lake.
"It's like looking at trees rings," said Heyvaert, who received his undergraduate degree in molecular biology from the University of Colorado, Boulder. "You can go back and reconstruct historical patterns."
Right now Heyvaert is working on two studies which are key to the "Total Maximum Daily Load," and massive research project to determine how much sediment and nutrient Lake Tahoe can absorb without losing clarity.
Heyvaert is analyzing the effectiveness of Best Management Practices, and what impact stormwater has on the lake.
"Some (BMPs) work well, some don't," Heyvaert said. "If we are able to implement appropriate measures for controlling nutrient and sediment loading to the lake, we expect the lake to renew itself within 10 to 20 years."
Matthew Graham, Erosion Control Team leader for the Tahoe Regional Planning Agency, said BMPs are an evolving technology getting more effective every year. Particularly when they are designed for large parking lots or drainage along a highway.
"We now have the ability to actually filter the water," Graham said. "In the late 1990s, all we had was gravity separation. You put stormwater in a vault and held it there and the coarser sediment would drop out. Now we have filtration cartridges to filter specific (things) like nitrogen."
