Unprecedented Drought, Snow and Rain Hurt Clarity Levels
by | Jun 13, 2018
Historic drought followed by record-breaking precipitation and warm lake temperatures converged to produce the lowest annual average clarity levels recorded at Lake Tahoe in 2017, indicates data released by the Tahoe Environmental Research Center at the University of California, Davis.
The average annual clarity level for 2017 was 59.7 feet. This was a 9.5-foot decrease from the previous year, surpassing the previous lowest value of 64.1 feet in 1997. While mid-lake clarity levels can swing widely from season to season and year to year, the five-year average lake clarity is about 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,” said TERC Director Geoffrey Schladow, a professor of engineering at UC Davis. “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.”
Click here to read the entire 2017 State of the Lake report. TLOA is a proud sponsor of the annual report and TERC.
Clarity is typically at its worst in the summer, and tends to improve during the fall and winter months. In 2017, a year when nearly every corner of the globe experienced extreme weather events, that pattern diverged with the worst clarity extending through the fall.
Clarity values through mid-March were better than in many recent years. However, they failed to improve during fall and winter. Water Year 2017 (October 2016-September 2017) was California’s second wettest and Nevada’s seventh wettest in a 122-year record, with the Tahoe Basin seeing up to 300 percent of its normal precipitation. An additional early winter storm in November 2017 added an unusual amount of sediment that impacted fall clarity.
Most of the sediment load in the low-clarity year of 1997 came in January with the peak flow. But 2017 produced a far greater load of sediment than the lake experienced in 1997, and those loads came more frequently throughout the year.
The year 2017 marked the end of the California’s worst drought in at least 1,200 years, followed by one of the wettest winters on record, when more than 50 atmospheric rivers emptied over the region.
In 2017, heavy rains flushed sediment that had been accumulating in streambeds for five years during the drought into the lake. Algal concentrations have been shown to reduce clarity in past years, but those were not significantly higher in 2017 than in other years. This indicates that sediment particles were likely responsible for the overall clarity decline.
Clarity is measured as the depth to which a 10-inch white disk, called a Secchi disk, remains visible when lowered into the water. In 2017, 26 individual Secchi depth readings were taken from January through December. The highest value recorded in 2017 was 90.2 feet on March 9, and the lowest was 47.6 feet on Oct. 17 and Dec. 19.
While both average winter and average summer clarity values decreased in 2017, neither were the lowest on record. The average winter value was 76.4 feet, which is 10.8 feet above the worst recorded level of 65.6 feet in 1997. The average summer value was 53.5 feet, which is 3 feet above the lowest recorded level of 50.5 feet in 2008. However, fall readings for clarity were the lowest on record, causing the overall average to drop.
Summer temps warmest on record
Summer temperatures in 2017 were the warmest on record at Lake Tahoe, almost 3 degrees Fahrenheit higher than in the previous three years. Unusually low wind speeds in July and August caused elevated water temperatures to extend into September, later than normal for the lake. Past research has predicted that warming lake temperatures would hold fine sediment particles closer to the surface longer, reducing clarity.
Schladow said that while anomalous years such as 2017 can at first appear alarming, they serve the important role of teaching us where future research is required to build resilience to climate change.