When I was growing up in the Central Valley of California, hot, dry summers were punctuated by an annual family vacation to Lake Tahoe. My parents would pack the '67 Dodge Coronet station wagon with the faux-wood paneling full of coolers, suitcases, and sundry beach equipment, and we'd head east on Interstate 80, up and over Donner Pass and, finally, into the narrow basin that surrounds the lake.

Most years we'd leave the beachfront just long enough to rent a boat and explore the lake from its own surface. Motoring roughly southeast from the north shore, we'd pass over a clearly visible line in the lake surface. On the shore side, the water was bluish, and if you couldn't actually see the bottom, you at least had the feeling it wasn't too far down. On the far side, the water was ominously dark where the lake bottom, already more than a hundred feet deep, dropped precipitously another 1,000 feet. But in recent years, I am told, this grand demarcation of the north shore shelf has become less visible and the water less blue. In fact, Charles Goldman, founder and director of the Tahoe Research Group (TRG), says the water clarity in this cold alpine lake has dropped by a third in the last 40 years, from more than 100 feet of visibility to just over 60. And if anyone should know how the water quality has changed, it's Goldman.

As a newly appointed professor at the University of California at Davis (UC Davis), Goldman started his research at the lake in 1958 and has been working there ever since. For much of that time, his work, and that of others who have since joined the TRG, has focused on the lake itself, measuring its clarity, the rate of algal growth, and the water's nutrient levels. In the last decade or two, as the scientists realized that the water quality was intimately connected to the health of the adjacent watershed, the TRG's studies have expanded to include issues of the basin's ecology.

Surrounded on all sides by steep, rocky mountains, Lake Tahoe sits in the bottom of a basin, and, in its natural state, it is a relatively sterile lake with few nutrients available to support algal growth. The basin comprises the lake's entire watershed, which is small compared to the surface area of the lake. Thus, unlike the watersheds of more eutrophic lakes, the water that flows into Tahoe doesn't travel far enough to collect large amounts of nutrients on the way, especially since much of the adjoining land is rocky and, therefore, not a rich source of nutrients.

The low density of algae means the lake waters are transparent, displaying the cobalt blue color that spawned much of the lake's fame. But Goldman says that increasing human activity has increased the flow of nutrients into the lake, allowing the algae to flourish. As a result, the water is changing, growing steadily cloudier and showing a tint of green.

For optimal growth, algae require a ratio of 105 carbon atoms to 15 nitrogens to one phosphorous; in the early sixties, Goldman found that the amount of nitrogen in Lake Tahoe's water was limiting. If he added more nitrogen to water samples, the algae bloomed. Meanwhile, he says, a group of civil engineers were trying to decide what to do with the sewage generated by the ever-growing population around the lake. The lore at the time was that the lake was so deep and so cold that the water at the bottom of the lake never mixed with the water in the upper layers; so one idea floated by the engineers was to pump the sewage to the bottom of this very deep lake, where it would never be seen again - or so they presumed. But when Goldman did some preliminary tests to check the viability of this option, he found that the amount of oxygen in water samples taken from relatively deep in the lake indicated that there is significant mixing. And of more immediate importance, he showed that adding even a small amount of nitrogen-rich sewage to lake samples generated enormous amounts of algae. As a result of these data, the engineers decided to pump the sewage out of the Tahoe basin altogether.

Though Goldman was able to help avert a near disaster by blocking this ill-conceived sewage scheme, the TRG has subsequently found that more subtle sources of nitrogen are contributing to the eutrophication of the lake. For example, as the surrounding shoreline becomes more and more urban, air pollution, which is nitrogen rich, has increased. In fact, said Goldman, "The air quality in a casino parking lot [on the south lake shore] is not that dissimilar from the air in Los Angeles." And this is in a region Mark Twain described as the "fairest sight the whole Earth affords." Additionally, much of the wetlands that once surrounded the lake have been filled in. This means that nitrogen-containing sediment, which was once filtered out of streams before they reached the lake, now flows directly into it. Together these changes have sufficiently altered the nutrient balance in the lake so that nitrogen is abundantly available, and phosphorous has become the limiting factor in the rate of algal growth.

Until relatively recently, TRG scientists presumed that algae bloom was the major factor in the loss of water clarity, but now they find that sediment accounts for nearly 45 percent of the opacity. Again, human activity seems to be a major cause of the problem. Disruption of the natural ground cover, road dust, and an increased rate of runoff from rain and snowmelt means more sediment is carried into the lake.

To minimize the effects of development and preserve the natural beauty of the region, Juan Palma, executive director of the Tahoe Regional Planning Agency, says that land use plans and public policy in the area are driven by the scientific data TRG has gathered over the years. "If you ask people why they moved to Tahoe, 90 percent of them say it's because of the environment," said Palma. So while many residents were resistant to such ideas when Goldman started sounding the alarm in the 1970s, most people now are accepting restrictive policies regarding land use and construction, which are designed to preserve the lake.

But will the land-use restrictions and the planned watershed restoration be adequate to restore the lake to its original clarity? The researchers and Palma admit that they don't know for sure, but they are developing mathematical models to find out. After all, "We don't have 30 years to wait and see if what we're doing is enough," says Palma. "We need to see it today."

The TRG gained valuable support and visibility in 1997, when President Bill Clinton and Vice President Al Gore visited the research team, even joining them for a trip onto the lake on the TRG research vessel. Prior to leaving office, in the fall of 2000, Clinton acted on his enthusiasm for the project and signed a bill allocating $300 million dollars for the reclamation of Lake Tahoe and its watershed, a third of the money required to complete the project.

The publicity generated by the presidential visit has also changed the science at TRG. What was once a project dominated almost entirely by researchers from UC Davis now includes collaborators from all over the world. John Reuter, one of the main members of the Davis team, notes that as the work expands beyond limnology (the study of freshwater lakes), it is important for collaborators to bring in new skills and training.

Also, Reuter says a number of the researchers are interested in getting involved with TRG's work because the study is unique. Most scientists studying ecology and environmental processes pick a process and go where it takes them. Instead, Goldman has picked a study site and examined many of the processes influencing it and has done so for nearly 40 years. Data sets covering that expanse of time simply don't exist elsewhere. "When I joined the group in 1978," said Reuter, "the data set was one of the few long-term data sets in the country, which was 20 years then." Now other groups may have 20-year data sets, but Goldman's work "was the pioneering work," says Reuter.

He also points out that the long-term study allowed the scientists to see the changes as they were happening and may give them an advantage in the reclamation project. "It helped us to get prepared," said Reuter. "Though we didn't know this day would come, it helps us so we're not starting from square one."

Will the lake recover? For an answer, Goldman looks to a recent set of experiments analyzing core samples from the lake bottom. In these samples, the researchers can see changes in sedimentation rates that correspond to periods of disruption and calm in the basin. In the late 1800s, when nearly all of the trees in the area were clear-cut to supply wood to the Comstock Silver Mine in Nevada, the lake sediment rate was very high. But the lake recovered during the 1920s and 1930s, in what was referred to as a heyday for the lake in terms of its clarity. Based on these data, both Reuter and Goldman think that the lake can overcome the current problems, as long as the sources of the contaminants are controlled. Goldman says, "I'm guardedly optimistic for the first time in years. I don't think we can ever take the lake back to what I encountered in the 1950s, but, it is still one of the clearest large lakes in the world." Showing his attachment to his research subject, Goldman continues, "In my view, Tahoe is still a really magnificent lake."