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!
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