Wildfire smoke might have decreased lake clarity

Wildfire smoke might have decreased lake clarity

20 March 2009

published by www.tahoedailytribune.com

USA — Despite a summer in which Western wildfires pumped smoke into the Lake Tahoe Basin, Lake Tahoe’s waters remained nearly as transparent in 2008 as they were in 2007.

Lake Tahoe was clear to an average depth of 69.6 feet in 2008 — a measurement that mirrors yearly averages from the past decade — according to data from the UC Davis Tahoe Environmental Research Center. The clarity figure for 2007 was 70.1 feet.

But researchers found the lake was unusually cloudy from mid-July to mid-August, and lingering smoke from regional wildfires could be the culprit.

The typical depth in that summer period from 2003-2007 was 50 feet to 65 feet, but in 2008, the depth was 36.9 feet to 46.8 feet, according to a statement from the research center.

Wildfire smoke for several weeks in summer dropped ash onto Lake Tahoe and could have dirtied the lake and fed algae growth, said John Reuter, associate director of the Environmental Research Center in a statement.

The overall summer clarity depth — 50.5 feet — was also the shallowest measured since monitoring began in 1968.

Although the wildfires that might have affected Lake Tahoe’s clarity over the summer were farther from the lake than the 2007 Angora fire, their effect on clarity could be much greater because of the extended period that smoke stayed in the basin, said Environmental Research Center Director Geoff Schladow.

Smoke from the Angora fire was quickly carried out of the basin and had a “small but measurable” effect on Lake Tahoe’s clarity, according to UC Davis researchers.

Scientists will analyze data collected this summer to quantify regional wildfires’ effect on Lake Tahoe’s clarity and hope to have results in about a month, Schladow said.

Although fine particle runoff from urban areas has been pinpointed as the most significant contributor to Lake Tahoe’s clarity loss, regional wildfire’s possible effect show how complex the clarity issue can be.

“What 2008 highlighted is the impact that wildfires and other factors outside our direct control can have on Lake Tahoe,” Schladow said. “While progress is being made in both understanding and addressing the root causes of clarity decline, the path to achieving the desired clarity will not be a straight one.”

Last year, UC Davis researchers reported that data since 2001 suggests clarity loss is slowing.

Historically, Lake Tahoe lost about a foot of clarity a year. The clarity loss is now believed to be a matter of inches per year, Schladow said.

The clarity reading from 2008 appears to uphold this trend, Schladow said.

Three consecutive dry winters have certainly helped recent numbers, but Schladow said there’s more to the decreasing clarity loss than just a lack of water runoff to push sediment to the lake.

“It is evident that the rate of decline decreased since 2001,” Schladow said. “Clearly, it will get a little worse if its a wet year, but I don’t think it will return to the historic trends.”

One year’s clarity measurement does not hold much significance, but the fact that Lake Tahoe’s clarity decline appears to be slowing is encouraging, Schladow said.

The sentiment was echoed by Tahoe Regional Planning Agency officials who coordinate the Environmental Improvement Program — a public-private partnership to restore and preserve the Tahoe Basin ecosystem.

“We are encouraged that the long-term trend showing clarity loss slowing has held,” TRPA Executive Director Joanne Marchetta said in a statement. “Despite the apparent impact seen from wildfires outside the basin last year, Lake Tahoe’s future appears hopeful.”

How is Lake Tahoe’s clarity measured?

UC Davis researchers measure Lake Tahoe’s clarity with a white, dinner-plate sized disk —known as a Secchi disk — every seven to 10 days at two fixed locations.

The disk is lowered into the lake and the depth at which the disk disappears from sight is referred to as the Secchi depth. The depth readings are averaged to give a yearly clarity number.

Individual 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.

When measurements began in 1968, the lake was visible at an average depth of 102.4 feet.

Clarity by the year

2008: 69.6 feet

2007: 70.1 feet

2006: 67.7 feet

2005: 72.4 feet

2004: 73.6 feet

2003: 71 feet

2002: 78 feet

2001: 73.6 feet

2000: 67.3 feet

1999: 69 feet

1998: 66 feet

1997: 64 feet

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