Lake Tahoe clarity decreased 9.5 feet in 2017 | SierraSun.com

Lake Tahoe clarity decreased 9.5 feet in 2017

Tahoe Daily Tribune

Citing the convergence of drought, record precipitation and warm temperatures, researchers announced Wednesday that Lake Tahoe's famed clarity declined 9.5 feet in 2017.

The numbers from the UC Davis Tahoe Environmental Research Center follow a 3.9-foot decrease in average annual clarity in 2016. The decrease in 2017 dropped the average annual clarity level to 59.7 feet.

Although the measurement plummeted past the previous lowest recorded average of 64.1 feet, which was recorded in 1997, officials pointed out that clarity can swing greatly from year to year and from season to season.

The five-year average lake clarity is approximately 70 feet.

"In 2017, Lake Tahoe's low clarity was primarily the result of two extreme climatic and hydrologic events — a perfect storm, so to speak," Tahoe Environmental Research Center Director Geoffrey Schladow, a professor of engineering at UC Davis, said in a statement. "The combination of arguably the most extreme drought period ending with the most extreme precipitation year produced the low clarity values seen. Measurements for 2018 have already shown a large improvement that are more in line with the long-term trend."

The impact of years drought followed by one of the wettest winters on record is reflected in last year's seasonal clarity shifts, or lack of shifts, researchers said.

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Typically clarity is worst in the summer months before improving in the fall and winter months. While clarity through mid-March fared better than in previous years, it failed to improve in the fall and winter.

Researchers say this is due, in part, to the volume and increased frequency that sediment was dumped into the lake. In 1997, which also saw a heavy-precipitation winter, peak sediment flow came in January

That was no the case in 2017, which marked the end of marked the end of California's worst drought in at least 1,200 years.

According to the research center, "2017 produced a far greater load of sediment than the lake experienced in 1997, and those loads came more frequently throughout the year."

That point was further illustrated by the fact that algal concentrations — another leading cause of reduced clarity — were not significantly higher in 2017 than other years.

Researchers also cited warming temperatures as an ingredient in the low-clarity cocktail. According to the research center, 2017 summer temperatures were the warmest on record at Lake Tahoe — nearly 3 degrees warmer than in the previous three years.

Warming lake temperatures can hold fine sediment particles closer to the surface longer, reducing clarity, according to previous research.

Average Lake Tahoe Secchi Depth

Annual Winter Summer

Year (feet) (feet) (feet)

1968 102.4 109.6 94.2

1969 93.8 119.1 74.8

1970 99.1 99.4 93.5

1971 94.2 109.9 86.3

1972 89.9 85.6 91.2

1973 85.6 96.8 75.1

1974 89.2 97.4 83.0

1975 85.6 94.5 77.8

1976 89.9 90.6 84.6

1977 91.2 91.2 92.8

1978 85.0 87.6 82.0

1979 87.6 95.1 81.7

1980 81.4 90.9 74.8

1981 89.9 81.7 97.8

1982 79.7 90.6 64.6

1983 73.5 95.1 57.1

1984 74.8 72.2 74.5

1985 79.4 89.6 72.5

1986 79.1 88.3 74.1

1987 80.7 76.1 85.6

1988 81.0 77.4 91.9

1989 77.4 87.6 75.5

1990 77.4 84.6 75.5

1991 73.5 70.9 72.8

1992 78.4 72.5 82.7

1993 70.5 84.6 65.3

1994 74.1 71.5 77.8

1995 70.5 75.1 58.1

1996 76.8 88.3 69.2

1997 64.0 65.6 62.7

1998 65.9 76.1 59.7

1999 68.9 81.0 63.0

2000 67.3 70.5 64.0

2001 73.5 77.8 72.8

2002 78.1 78.4 81.0

2003 70.9 70.9 69.2

2004 73.5 83.3 73.2

2005 72.2 80.4 66.9

2006 67.6 76.8 57.4

2007 70.2 82.3 65.3

2008 69.6 85.3 50.5

2009 68.2 81.4 59.1

2010 64.3 72.8 51.8

2011 68.9 85.0 51.5

2012 75.1 88.3 64.6

2013 70.2 77.8 63.6

2014 77.8 79.1 76.8

2015 73.2 71.5 73.2

2016 69.2 83.3 56.4

2017 59.7 76.4 53.5