The UC Davis scientists who study this stuff dangle the Secchi disk (basically a white dinner plate or a black-and-white plate) down off the shady side of a boat at mid-day. They lower it until they can no longer see it, at which point they note the depth. After they start pulling it back up, they again note the depth when the plate reappears. Often the two figures vary a bit, so they average them. And just to make sure that they are getting reliable data, they do this measurement many times during the course of a year.
The average for 2012 was 75 feet. The lake hasn't been that clear since 2002.
|UC Davis's John Le Conte research vessel|
Before we get too excited and smug, it is good to remember that when they started taking these measurements in 1968, that little Secchi disk could be seen 102 feet down!
While this improving trend is great, the reasons why it is happening are less clear. At this point, the best guess is that the main mitigation has come from all of the infiltration ponds that have been built to catch and filter runoff water from streets as well as rebuilding creeks that once had meandering paths and flood zones but were dredged and straightened by developers in years past.
There is lots more to do. There is still a scary number of drainage pipes that dump dirty storm runoff water directly into the lake. These have been documented by the Tahoe Pipe Club.
There are infestations of non-native mussels and fish that lead to algae blooms. There are massive ongoing erosion areas from old road cuts such as Meyer's Grade.
And there are other problems that might be even harder to tackle, such as the nutrient load from dust and dirt that blows in from the Central Valley, especially when the farmers are plowing or burning slash. Scientists have even identified silt that has blown into the lake from China's and Mongolia's Gobi Desert after a dust storm half a world away.
But for now, we are glad that the lake is improving, and we salute those individuals and groups that made it possible!