Scientists have been measuring water clarity in Lake Tahoe continuously since mid-1967. This simple measurement of how far down into the lake a white Secchi Disk can be seen tells lake specialists, called limnologists, a great deal about a lake’s ecological condition.
Dr. Charles Goldman, founder of the UC Davis Tahoe Research Group, is the limnologist who had the foresight to begin scientifically accurate measurements of Lake Tahoe’s clarity thirty-five years ago. Because of this, and only because of this, we know that the Lake’s clarity has fallen at an average of over a foot a year, from approximately 105 feet in 1968 to around 70 feet in 2002.
These measurements are summarized in the famous Lake Tahoe Secchi Depth chart, as shown. [insert Secchi Chart]
Unlike most graphs, this chart shows the measurements reading down from the top. This corresponds with the fact that the water clarity is the distance down into the lake that Bob Richards can see the disk from the deck of the UC Davis research vessel, the John LeConte. Note that while the Secchi depth fluctuates depending on the weather, storm patterns and amount of precipitation each year, the trend is unmistakably toward lower clarity over the years. The rising trend line indicates decreasing Secchi depth.
Water clarity is an indicator of the ecological condition of a lake. When Mark Twain spoke of his amazement at seeing objects clearly on the bottom of Lake Tahoe in over a hundred feet of water, he was describing what a rare circumstance it was even 130 years ago to encounter a lake this clear. The lake was then, as it is now, an “oligotrophic” lake. That means it is exceptionally clear because there is very little suspended sediment and very little suspended algae in the water. Such lakes tend to be naturally clear, because their watersheds are relatively small compared to the volume of water in the lake.
What the Secchi chart is telling us is that we are destroying the Lake’s natural clarity. Our urbanization of Tahoe’s watershed and our pollution of the air quality over the lake have created millions of small human impacts that have added up over time. These cumulative impacts have caused more nutrients to enter the Lake and to feed the free floating algae, and more small soil particles to become suspended in the water. Charles Goldman is still working on Lake Tahoe. He is telling us that the water quality may move from spectacular to merely average in our lifetimes, with the color changing from blue to green. Restoration of our air quality and watershed represents the only likely means to insure that this change does not go too far.
Lake Tahoe is still one of the clearest alpine lakes in the world. Many people think we should do whatever we can to keep it that way. They argue this is not just important for aesthetic reasons, but also for Tahoe’s recreation-based economy and to keep property values from declining over time. Moreover, many Tahoe communities and also Reno and Sparks get most of their drinking water from Lake Tahoe. If we allow the Lake to become ever more polluted, the costs of treating our drinking water will continue to escalate.
The Lake Tahoe Report 003
Air Date: 2003.02.18
Video Segment: Watershed and Water Clarity at Lake Tahoe
Interviewees: Bob Richards (TRG)