Water clarity is measured by watching a white, dinner-plate sized disk as it is lowered in the water. The depth that it disappears is recorded, along with the depth at which it can be seen again as it is raised. The average of those two measurements is called the Secchi depth. This technique was invented by Professor P.A. Secchi for his research on the Adriatic Sea. It's deceptively simple, but very repeatable, and has been a standard technique for oceanographers and limnologists for 133 years.
Clarity varies from year to year, depending on algal growth and suspended sediment in the lake.
For example, clarity decreased by some six meters between 1968 and 1973. Yet, from 1973 to 1980, there was very little change. Clarity dropped from 1980 to 1985 and then was relatively unchanged for the next five years. If we had measurements from just one of those periods, we might have concluded that there is no change in clarity or that clarity is decreasing much faster than it actually is. Without the long term record we now have, we really wouldn't know how the lake is doing.