California's water supply depends on a clean snow pack and healthy mountain lakes.  Because of geography, California cities are conducive to air pollution and a similar effect can be found in lakes. The lakes in the Sierra Nevada are the most sensitive lakes in the U.S. to acid rain, according to the Environmental Protection Agency.

The lakes receive a large amount of runoff in the spring from the melting snowpack. If the snowpack is polluted, the lakes will be polluted.

Carbonaceous particles, formed by burning oil and coal, are an indicator of air pollution and acid rain. They are transported, along with sulfuric and nitric acid, through the atmosphere and deposited in the lakes. Since 1970 there has been declining levels of carbonaceous particles in the Sierra Nevada lakes, indicating that air quality in California (and likely the western U.S.) has improved dramatically. The authors attribute that to legislation, notably the 1955 Air Pollution Control Act signed by President Eisenhower and the U.S. Clean Air Act signed in 1963 by President Kennedy. Major updates have been signed by Presidents Nixon, Carter and Bush.

"Without the Clean Air Act, air quality in Los Angeles and the Central Valley would be much, much worse increasing cases of asthma and other respiratory diseases. The Clean Air Act should be emulated by China and India where acid rain and air pollution are ruining the environment and making people sick," said James O. Sickman,
a professor of hydrology
 at the University of California, Riverside.

Less acidity and lower nitrogen inputs have allowed sensitive aquatic species to be maintained in the Sierra Nevada lakes. For example the lakes still possess native clams, zooplankton, sponges and invertebrates that could have been eradicated by higher acidity. Another impact has been the preservation of water clarity and color. Greater acid rain and nutrient input would have encouraged algal growth, clouding the water and impairing the lakes' deep blue color. Less acidity in mountain lakes preserves these fragile ecosystems and maintains their visual beauty.

To do the research, Sickman and colleagues collected sediment cores from about 50 lakes in the Sierra Nevada. The researchers hiked to the lakes and used rafts to access the deepest part of the lake. They lowered corers into the sediments. When retrieved, the sediment cores were cut into 1-centimeter-thick slices.

The slices were then subjected to chemical analyses to determine the age of the individual slices. The researchers then counted the carbonaceous particles and diatoms (small phytoplankton with silicon bodies) under a microscope.