In the 1960s, a white disc resembling a dinner plate used to measure Tahoe's clarity was visible at depths of 105 feet. Today, that same plate is visible only as deep as 66 feet, and might be seen only as deep as 40 feet by the year 2030 if current trends continue, the report said. And it's going to take some dramatic measures to turn things around. Cahill said more than half of the nutrients come from the air. Among the sources are wood smoke, diesel exhaust and sand that is applied to roads in winter, then gets ground into a fine powder that winds up in the lake when snow melts.
Construction is also to blame.
In 1960, just 500 houses ringed the lake. Twenty years later, there were 19,000 homes. Runoff from development enters the lake through ground water and tributaries that feed the lake. Scientists say that unless urgent measures are taken now, the lake will become stratified within 30 years, with a warm algae-rich layer on top and a cold, sterile layer below it. Once stratification happens, it's virtually impossible to reverse.