There is something ineffable about trying to measure a thing by the point at which it is no longer measurable. But that was exactly what we were doing, leaning over the side of a boat, watching a white plate sink into Lake Tahoe. We counted off the feet on the line attached to the Secchi dish, named for Angelo Secchi, a 19th-century scientist at the Vatican, who invented this method to measure water clarity in the Mediterranean.

"10, 20, 30."

Sunlight reflected off the white surface and refracted through the blue water, casting a crystalline halo around the dish. Then the plate began to lose definition around the edge. Soon it was just a pale diffuse
light, getting smaller and smaller in the darkening depths.

"40, 50, 60."

"I don't see it anymore," I called out. The plate had descended to a depth where, technically speaking, the difference between the wavelength of the light bouncing off of it and the wavelength of the light being reflected by microscopic particles of sediment in the water had become smaller than my eyes could detect. The Secchi dish was still down there, reflecting light back up to the surface. I just couldn't swear to it.

But Brant Allen could. "I still see it," said the crew-cut captain of the RV John LeConte, the research vessel that had brought us out on the lake this bright morning. I counted as he continued to slowly play out the line.

"64, 65, 66, 67."

"Now I don't see it either," said Allen, a fisheries biologist who has recently taken over this tradition, carried out regularly for 39 years by scientists from the University of California at Davis. Allen began reeling the dish back in and soon it came back into focus, a tiny source of light now differentiated again from the scattering luminescence of the deep blue lake.

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.

This simple measurement has been a key to a multimillion dollar drive to stop the decline in clarity and "Keep Tahoe Blue," as a ubiquitous local bumper sticker proclaims. The Tahoe Regional Planning Agency has 36 threshold indicators that guide its regulations from "atmospheric nutrient loading" to "phytoplankton primary productivity." None of these measurements has the emotional and political power of the Secchi depth -- except, that is, for the one that is not even on the list: keeping Tahoe blue. The problem with that slogan, of course, is: Which shade of blue are we talking about?

 In 1873, John LeConte, the scientist for whom our vessel was named, wrote about this dilemma. "On a calm, clear, sunny day, wherever the depth is not less than from fifty to sixty meters, to an observer floating above its surface, the water assumes various shades of blue, from a brilliant Cyan blue ... to the most magnificent ultramarine blue or deep indigo blue," he wrote in the Overland Monthly, a popular Western magazine. "While traversing one portion of the Lake in a steamer, a lady endowed with a remarkable natural appreciation and discrimination of shades of color declared that the exact tint of the water at this point was 'Marie-Louise blue.' "

 LeConte was a bird-watcher and a doctor. He was interested in the transmission and perception of color and light. But he was mad for quantification, too. So he took the fateful next step. "On the 6th of September, 1873, the writer executed a series of experiments with the view of testing the transparency of the water," he wrote. "By securing a white object of considerable size -- a horizontally adjusted dinner-plate about 9.5 inches in diameter -- to the sounding-line, it was ascertained that (at noon) it was plainly visible at a vertical depth of 33 meters, or 108.27 English feet."

Why does "108.27 feet" sound more accurate than "Marie-Louise blue" -- besides the fact that the imperial naming of colors has gone out of our common lexicon? Both measurements are subjective; they depend on the eyes of the beholder. And while we demand the precision of numbers, we need the story that surrounds them, too. It is the story of keeping Tahoe blue that gives the numbers power.


Jon Christensen has taught courses on the history of science and the environment in the University of Nevada, Reno's Interactive Environmental Journalism graduate program, which focuses on Lake Tahoe and produces