Water in Lake Tahoe fully flips for first time since winter of 2018-19

LAKE TAHOE, Calif./Nev. - Lake Tahoe flipped on February 27 or 28 according to the UC Davis Tahoe Environmental Research Center (TERC). Flipping is when the water fully mixes vertically from top to bottom.

Even though smaller lakes are fully mixed annually, it is less common for Lake Tahoe with a depth of 1,640 feet. The last time Tahoe flipped was in the winter of 2018-19.

TERC researchers are on the like every week to sample water quality, the phytoplankton, and the overall health. They maintain instruments placed on the lake that take instruments every few minutes.

Why is fully mixing a good thing? Full mixing renews the water at the lake bottom with “fresh” oxygen-rich water from the surface. Oxygen is constantly being lost from the lake bottom, so it requires replenishment. Mixing also helps cool the bottom of the lake, which slowly warms due to geothermal heating.

TERC reports that typical mixing starts in the fall, with the surface layer of the lake cooling and gradually mixing deeper. In most years, the mixing does not extend beyond 1,000 feet. The figure above shows the change in temperature with depth on days in December, February, and March. Note how the cooling of the lake surface progresses, and how the cool water extends down deeper and deeper. On February 1, the lake had only mixed to 500 feet. In less than 4 weeks, it had mixed a further 1,100 feet. On March 3, the entire lake was essentially the same temperature from top to bottom.

In the second photo above, the figure shows the temperature since February 10 at the lake surface from a buoy and at the bottom of the lake (41.34 oF measured on February 1). The daily spikes in the buoy data are due to sunlight warming the surface of the lake during the day. On February 28, the two temperatures were equal, and the entire lake started freely mixing from top to bottom. In March, researchers will retrieve an instrument deployed at the lake bottom near Glenbrook and will have a minute-by-minute description of events at the bottom during mixing.

What causes the mixing? Surprisingly the air temperature is the largest factor, not the intensity of individual storms. This has been a particularly cold winter, causing the lake to mix deeper and weeks earlier compared to most other years.

Since the deepest water in the lake is also the clearest, mixing gives the surface water a short time of high clarity.

This year, two days after mixing, the Secchi depth was an astounding 115 feet, almost 33 feet deeper than it had been a week earlier. The Secchi disc has been used at Lake Tahoe to routinely monitor clarity since 1968, the best being 142 feet on February 8, 1968.

The mixing also redistributes nutrients. Algae and organic material in the lake eventually end up at the bottom, and through decomposition, nutrients are released. These nutrients can build up over many years, so when deep mixing takes place, the bottom nutrients are carried all throughout the lake. In some years this is the largest source of nutrients to the lake surface and can lead to increased algal growth as well as a decline in lake clarity. In the coming months, we expect clarity to decrease as algae grow and fine particles begin entering the lake with the snowmelt.

A big shout out to the TERC field team who are out in all seasons collecting measurements and deploying instruments in all kinds of weather, and to the UC Davis data impresario Dr. Shohei Watanabe who waits eagerly for each piece of new data. The temperature data collection was in part funded by the Tahoe Regional Planning Agency. The temperature buoys are operated in collaboration with the NASA Jet Propulsion Laboratory.