UC Davis: The trend at Lake Tahoe is toward clarity | The Sacramento Bee

The latest scientific findings from Lake Tahoe show a trend toward long-term water clarity.

Clarity levels at the alpine lake continued to stabilize in 2013, according to University of California, Davis scientists. The average clarity reading for last year was 70.1 feet.

Even though that reading was 5 feet less that the previous year, it is much better than the reading recorded in 1997 of just 64.1 feet and above more recent years’ averages.

The clarity level is arrived at by taking the average of 25 readings. The best reading last year was 90 feet, the worst 49 feet.

“Clarity in Lake Tahoe largely reflected what we saw in the weather in 2013,” said Geoffrey Schladow, director of the UC Davis Tahoe Environmental Research Center. “At the beginning of the year, clarity was lowered by large stream inflows. At the end of the year, the low inflows resulting from the drought conditions helped to improve clarity.”

Water clarity is measured by the depth at which a 10-inch disk lowered into Lake Tahoe can been seen. Such measurements have been taken since 1968 when the disk could be seen down to 102.4 feet, according to a UC Davis press release.

Scientists say the average annual clarity in the past decade has been better than in preceding decades, but it is still short of a target clarity of 97.4 feet set by federal and state regulators.

Urban storm water runoff is believed to be the major contributor to reduced clarity at the lake.

“Through the seasonal and annual fluctuations, the long-term clarity trend is good news, and it tells us that the investments being made on roadways and properties to infiltrate stormwater are working,” said Tahoe Regional Planning Agency executive director Joanne Marchetta.

Clarity readings through the years:

2013: 70.1 feet

2012: 75.3 feet

2011: 68.9 feet

2010: 64.4 feet

2009: 68.1 feet

2008: 69.6 feet

2007: 70.1 feet

2006: 67.7 feet

2005: 72.3 feet

2004: 73.6 feet

2003: 70.9 feet

2002: 78.0 feet

2001: 73.6 feet

2000: 67.4 feet

1999: 69.0 feet

1998: 66.1 feet

1997: 64.1 feet