Science raises hope for Tahoe restoration
As recently as a decade ago it was widely thought that it could take as long as seven centuries to restore Lake Tahoe’s clarity. This was based on calculations of how much water flows into and out of the lake. It was not very encouraging.
Then, some nine years ago, Alan Heyvaert, then a UC Davis scientist (now with the Desert Research Institute), carefully analyzed cores taken from sediments in the bottom of the lake. Lake bottom sediment layers form a kind of historic filing cabinet. They contain records of what was happening while the sediment was being deposited.
Heyvaert found that the lake took a heavy hit of sediments and nutrients during the Comstock logging activities of the late 1800s. He also found that during the next three decades or so, before development escalated around the Basin, the lake recovered. This showed that we don’t have to wait 700 years to see if our restoration efforts are successful. This was very encouraging, to say the least.
A few years later, UC Davis scientists Alan Jassby, John Reuter, and Charles Goldman examined the 37-year record of Tahoe’s water quality measurements along with historic climate records. They found a close correlation between annual precipitation and lake clarity. During years with low precipitation, less pollution is washed into the lake, and clarity improves. This showed that the lake can respond, at least to a small degree, very quickly. This was even more encouraging.
And now, even better news: Preliminary results from the Lake Tahoe Clarity Model show that we don’t need to control every bit of erosion and nutrients in the basin to restore the lake’s clarity. According to UC Davis scientists Geoffrey Schladow and John Reuter, something like a 35 percent decrease in the total annual load of nutrients and fine particles flowing to the lake would allow it to restore itself. And that might be accomplished by reducing the pollutant load by 1.75 percent each year for 20 years.
Restoration of the lake means increasing clarity to a Secchi depth of 102 feet. That’s the TRPA standard, based on lake water clarity as measured during the 1968-71 period. The Secchi depth is the depth at which observers can still see a white dinner-plate-size disk (the Secchi disk) as it’s lowered into the lake ” under ideal viewing conditions.
Secchi depths are currently around 70 feet.
The Lake Tahoe Clarity Model is something that scientists working at Tahoe have wanted for a long time. Still, the magnitude of the job of developing a realistic lake model for Tahoe far exceeded the available dollars and personnel. The situation changed radically during the famous visit to Tahoe by President Clinton in 1997.
One of the “deliverables” promised by the federal government during that visit was a lake clarity model. Schladow and Reuter submitted a proposal to develop that model and were successful in getting EPA support.
It was fortunate that Geoff Schladow was on the UC Davis faculty. He is professor of water resources and environmental engineering and is now also the director of the Tahoe Environmental Research Center. Schladow has some 20 years of experience developing hydrodynamic models for lakes around the world. And UC Davis researchers had some 40 years of observations and research in the Tahoe Basin.
It was a good combination, but they still needed, and fortunately obtained, a tremendous amount of help. Perhaps as many as 150 different researchers, working full or part-time from the Desert Research Institute, University of Nevada, UC Davis, U.S. Geological Service, Forest Service Southwest Laboratory, NASA, and a host of other institutions and consultants provided the information needed to develop and validate the model. The Lake Tahoe Lake Clarity Model incorporates models that other groups have developed ” of watershed processes, historic climate, atmospheric deposition, and ground water.
A few years ago, developing this model became the center piece of the Lahontan Regional Water Quality Control Board’s TMDL program, and they found funds necessary to bring the model to its current state. The TMDL program seeks to establish just how much of an annual pollution load the lake can stand without deteriorating. And, this clarity model will be a vital tool in making decisions about how much and where to reduce pollutant sources.
Schladow explains that the clarity model is a “process model.” This means that all known significant lake processes are accounted for. For example, the model involves the hydrodynamics of the lake, its thermal processes, algae growth, and light scattering from algae and mineral particles. It includes deposition from the atmosphere, flows from streams, overland flows from urban areas, and seeps from aquifers. The model developers have had to account for the depths that stream water seeks as it enters the lake, mixing produced by prevailing winds, and settling rates of particles. As knowledge of these processes improves in the future, the model can be altered to reflect it.
This clarity model is great news, and we’ll learn more about it next time.
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