TERC Boat Captain and participants observe the Secchi disc as it is lowered into the water to find the clarity reading for the day.

Water Clarity

Lake Tahoe is known around the world for its famed water clarity, eliciting awe from millions of visitors each year. UC Davis researchers have been monitoring the changes in lake clarity for the past 50 years using a Secchi disk (the white dinner plate). From the late 1960's through the turn of the century, there was a decline in clarity brought about by increased algae growth and the presence of very fine particles. Over the past decade we have seen the rate of clarity decline ease with winter clarity actually improving. Using technology beyond the dinner plate, researchers continue to refine their understanding of lake physics and ecology to determine the cause of annual clarity change, enabling management agencies to focus on the most pressing issues effecting Tahoe's clarity.

In 2014 the annual average Secchi depth was 77.8 feet (23.7 meters), an increase of approximately 7 feet over the previous year. It is important to understand the causes behind clarity change and to evaluate past actions and future investments. 

Click here for a table of TERC's Secchi Data since 1968

From 1968 to 2000 there was a near-continuous decline in lake clarity. There were several years at a time when things seemed to improve, but invariably the same trend returned. However, since 2001, clarity has consistently been better than the long-term trend would have predicted. It is still a small number of years and therefore difficult to draw conclusions about, therefore only with the commitment to long-term monitoring can we truly evaluate environmental changes over time.