 Artist ‘s rendition of a Secchi disk on its descent into the water. Graphic courtesy the author. |
The simplest test of clarity of a lake is to use a “pie plate on a string” or, officially, the Secchi disk. This 20-cm diameter metal or weighted plastic disk is usually quartered black and white. It is lowered into a lake and the depth at which it disappears is recorded. The Secchi clarity depth at Crater Lake was recorded in June of 1997 at 43.3 meters. At Waldo Lake the deepest Secchi depth was measured to be 41.3 meters in July 2001. This “eyeball” test of a lake’s water clarity greatly depends upon the sky conditions and the lake surface. If clouds obscure the sun or the lake is ruffled by wind, the disk will prematurely disappear. To put this test in perspective, many lakes with large watersheds seldom have Secchi depths of greater than 10 meters. Muddy water may have a Secchi depth of less than one meter. In this case, two world-class lakes are being compared and can be seen to be very similar in clarity. It should be noted, however, that Waldo Lake does not currently host a full-time research effort while Crater Lake does. It is very difficult to record the best Secchi clarity depths on a part-time basis. Visiting the lake for a day or two each month lessens the chances that the sky and lake surface conditions will be ‘perfect’ for viewing the disk.
