Lake Tahoe, from Nevada (wikimedia.org)
According to the Secchi disk recently dunked in Lake Tahoe, the lake's water clarity has improved. First used in the mid-1800s by Angelo Secchi, an Italian astronomer and scientific adviser to the pope, Secchi disks--simple white metal disks used to judge water clarity-- provide a basic metric to determine lake clarity. The disks are lowered in water until they disappear from sight; the depth at which they disappear is noted, then results are compared with previous tests from the same water body.
What are some of Lake Tahoe's results? 64.1 feet in 1997, which improved to 70.1 feet in 2013; compare those to 1967, when viewers could see the disk 105 feet below the surface.