Warmer Waters Contribute to Decline of Lake Tahoe Water Clarity in the 2015 Annual Report (UC Davis Press Release)

Quick Summary

  • Lake Tahoe clarity declines nearly 5 feet in 2015 over previous year, to 73.1 feet.
  • Warmer water and lack of snow in 2015 contributed to decline.
  • Decline not part of a long-term trend. Average clarity has hovered around 71 feet for past 15 years.

Clarity levels at Lake Tahoe in 2015 declined in both summer and winter, due in part to warmer waters, according to researchers at the University of California, Davis, who have studied the lake for the last half-century. 

Data released by the UC Davis Tahoe Environmental Research Center and the Tahoe Regional Planning Agency reported the average annual clarity level for 2015 at 73.1 feet. That is the depth at which a 10-inch white disk, called a Secchi disk, remains visible when lowered into the water. (Figures reflecting this change are available below)

This is a 4.8 foot decrease from the previous year but is still over 9 feet greater than the lowest recorded average of 64.1 feet in 1997.

The declines are not considered to be part of a long-term trend. They are part of the year-to-year variability that has always characterized conditions at the lake. The record indicates that Lake Tahoe’s long-term trend of decline ended about 15 years ago. Since then, clarity has hovered around a value of 71 feet.

Click here to read the complete press release

Read more about the history of the Secchi Disk at Lake Tahoe below

2015 Annual Secchi Data
2015 Winter Secchi Data
2015 Summer Secchi Data

Beyond the Secchi Disk

UC Davis Historical Data Set

The Secchi disk is one of the most common and well-known pieces of equipment used to measure water clarity. Since 1968, UC Davis scientists have been using the Secchi disk to collect clarity measurements in Lake Tahoe. The Secchi disk may not seem like a high-tech way to measure clarity, but it never fails, does not require batteries or recharging, is easy to use, is economical, and never requires technical support. However, there are also issues that arise when taking Secchi disk measurements. For example, readings depend on the eyesight of the person taking measurements as well as the weather. If it's windy, the surface of the water will be irregular, alternatively if it's sunny, the sunlight will be reflected off the surface, thus altering results. Multiple measurements are taken throughout the year to compensate for these discrepancies and because this instrument has been used since the 1960's, using Secchi disk measurements will continue the long-term data record that will remain constant even while technology advances. The Secchi disk is still being used as a reference for general water clarity. However, it was created more than one-hundred years ago and scientific equipment has since made great strides. Today, even instruments that measured clarity thirty years ago are obsolete and some of the instruments we use today did not even exist ten years ago.

Differences in water clarity are primarily caused by the presence or lack of dissolved substances, phytoplankton, and suspended particles in the water. The Secchi disk measures overall clarity but does not indicate what is limiting light penetration in the lake. For researchers at Lake Tahoe, the scientific value is found in knowing what are the factors limiting water clarity. Researchers are now using new and innovative equipment to understand what factors obstructing lake clarity. Researchers at the UC Davis Tahoe Environmental Research Center are now using new and innovative equipment such as a Laser Instu Scattering Transmissometer (LISST) that continually measures the sizes of fine particles as it is lowered through the water column. A Fluroprobe measures the amount of chlorophyll, what species of phytoplankton is utilizing the chlorophyll, and if individual organisms are dead or alive. Lastly, the Seabird CTD is a multi-faceted piece of equipment, which in addition to measuring conductivity, temperature, and depth (pressure), also has a Photosynthetic Active Radiation (PAR) sensor which uses light attenuation to show what depths phytoplankton can photosynthesize. Another attachment on the Seabird CTD is a transmissometer which is similar to the LISST measuring the abundance of fine particles in the water column. All of the aforementioned equipment is used together with the Secchi disk readings to determine Lake Tahoe's clarity and contributing factors to change over time and through the seasons. By knowing all this information we can better understand the issue surround Lake Tahoe's clarity and what we can do to remedy the situation. By better understanding of the causes of change we have an opportunity to do what is needed to maintain or even reverse the loss of clarity at Lake Tahoe.