Editor's note: A chart of 33 years of annual clarity measurements is provided at end. Lake Tahoe Clarity Falls Again After two years of improvement, the clarity of Lake Tahoe declined two feet last year, reports the UC Davis Tahoe Research Group. UC Davis researchers who have studied the lake for more than 40 years have warned that, if unchecked, the decline in transparency will turn Tahoe's famous cobalt-blue waters to green. Transparency for the year 2000, as determined from the average of about 35 measurements taken with a Secchi disk during favorable viewing conditions, was 67.3 feet, or 20.53 meters. In 1999 and 1998, transparency averaged 69.0 feet and 66.0 feet, respectively. In 1997, lake transparency reached a record low of 64.0 feet. In 1968, when UC Davis researchers began their groundbreaking clarity studies, the white disk used to assess transparency could be seen at a depth of 102.4 feet. UC Davis professor Charles Goldman, an international authority on lakes and the director of the UC Davis Tahoe Research Group, noted that while year-to-year clarity measurements have gone up and down in the past, the long-term trend is declining transparency. "We are beginning to see benefits from many changes being made in the Tahoe Basin, but as this year's clarity data show, the lake is still at risk," Goldman said. "It's encouraging that an environmental ethic has evolved among the people who work and live in the basin. And with substantial federal and state funds, we are applying our research to improve programs in development, transportation and roads management, erosion control, forest health, wetland restoration, stream improvement, fertilizer controls, and controls of invasive plants and animals." The Tahoe Research Group's water-clarity monitoring activities in recent years have been largely funded by a $110,000 annual grant from the Tahoe Regional Planning Agency (TRPA), the bi-state agency leading the cooperative effort to preserve and restore the Lake Tahoe environment. TRPA executive director Juan Palma said the scientific data provided by the TRG and other research programs inform critical resource-management decisions. "The scientific community tells us, for example, that Lake Tahoe probably won't respond positively to restoration efforts in the watershed for 20 to 30 years," Palma said. "Given that we only recently began to pick up the pace of investing in those efforts, we must remain committed to our long-term plan for changing the downward trend in water clarity." Year-to-year variations in Tahoe annual clarity data depend upon a combination of environmental factors. The most significant factor is the extent of annual mixing of the lake's deep waters. That mixing brings up nitrogen and phosphorus, which nourish the growth of green-colored algae. However, the long-term decline in clarity seen since 1968 is caused by increased nutrients and sediments reaching the lake from urbanization in the surrounding watershed as well as from air pollution. UC Davis has been working with local, state and federal agencies, the private sector, the University of Nevada, Reno, and the Desert Research Institute to gain the scientific knowledge needed to help improve the health of the Tahoe Basin. Currently, the campus is planning a world-class laboratory, the Lake Tahoe Center for Environmental Research. "If we are to reverse the deterioration of this unique natural resource, researchers must help policy makers and resource managers make science-based decisions. The new laboratory will give us the facilities to provide that essential help," Goldman said. The Tahoe Research Group web site can be found at http://trg.ucdavis.edu/. Here is a chart listing clarity measurements made by the UC Davis Tahoe Research Group since 1968. Published charts or graphics must credit UC Davis Tahoe Research Group. Year, followed by average clarity depth (feet) 1968 102.4 1969 93.7 1970 99 1971 94.3 1972 89.9 1973 85.5 1974 89.2 1975 85.6 1976 89.8 1977 91 1978 85.1 1979 87.6 1980 81.4 1981 89.8 1982 79.7 1983 73.4 1984 74.8 1985 79.4 1986 79 1987 80.9 1988 80.9 1989 77.5 1990 77.5 1991 73.6 1992 78.4 1993 70.4 1994 73.9 1995 70.4 1996 76.9 1997 64 1998 66 1999 69 2000 67.3