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

Lake Tahoe clarity decreased 9.5 feet in 2017

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 (TERC)  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," TERC 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.

This post will be updated.

 

Average Lake Tahoe Secchi Depth

AnnualWinterSummer
Year(feet)(feet)(feet)
1968102.4109.694.2
196993.8119.174.8
197099.199.493.5
197194.2109.986.3
197289.985.691.2
197385.696.875.1
197489.297.483.0
197585.694.577.8
197689.990.684.6
197791.291.292.8
197885.087.682.0
197987.695.181.7
198081.490.974.8
198189.981.797.8
198279.790.664.6
198373.595.157.1
198474.872.274.5
198579.489.672.5
198679.188.374.1
198780.776.185.6
198881.077.491.9
198977.487.675.5
199077.484.675.5
199173.570.972.8
199278.472.582.7
199370.584.665.3
199474.171.577.8
199570.575.158.1
199676.888.369.2
199764.065.662.7
199865.976.159.7
199968.981.063.0
200067.370.564.0
200173.577.872.8
200278.178.481.0
200370.970.969.2
200473.583.373.2
200572.280.466.9
200667.676.857.4
200770.282.365.3
200869.685.350.5
200968.281.459.1
201064.372.851.8
201168.985.051.5
201275.188.364.6
201370.277.863.6
201477.879.176.8
201573.271.573.2
201669.283.356.4
201759.776.453.5