August 4, 2008--Measuring Tahoe's blues (High Country News)
Technically, we were measuring the declining clarity of Lake Tahoe by the point at which clarity ceases. We saw things slightly differently, as everybody does. We could measure again in the same spot, and it would be different. The lake and the light are always changing. But we had a measurement we could agree on, within a few feet, for this particular time and place. And for scientists, that is important. Averaged, with a statistical variation calculated, and repeated every 10 days, this single number has become a measurement of the current health of the lake, its trend, and its prognosis for the future. In 1968, when Charles Goldman took the first regular measurement in this series, the Secchi depth was 102 feet. Since then, the plate has disappeared sooner and sooner, at a rate of about a foot each year, as runoff brings sediment from the surrounding land, and pollution filters down from the sky.
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