By TOM MOOERS | Moonshine Ink

This is Abby and her first Secchi Disk.

Abby is 8 years old and loves science. She loves Tahoe, too, so she’s pretty excited.


The Secchi Disk was invented by Angelo Secchi of the Papal Observatory back in 1865.  Legend has it that Secchi tied a dinner dish to a string in order to measure the clarity of the Mediterranean Sea in the area where he lived. It worked.

Nowadays, this method is used by lifelong limnologists and by citizen scientists — like Abby — to measure the clarity of Lake Tahoe. The process is actually quite simple: Lower the disk off the side of a boat until it is no longer visible. The longer the string, the clearer the water.

Sadly, the clarity of Lake Tahoe is diminishing. Over time, the string has gotten shorter, the disk lowered down a little less. In 1968, scientists could still see it at 100 feet. In 2017, scientists lost sight of the disk at only 59 feet down.

That’s a stunning erosion of an invaluable natural asset.

What can our generation do to preserve lake clarity for future generations like Abby’s?

For decades, much has been done to Keep Tahoe Blue — to stop the nutrients that feed the algae that cloud the lake. And, to at least some extent, it’s working.

LESS CLEAR: Secchi Disk research shows steadily decreasing clarity over the years in Lake Tahoe, although last year’s readings saw an improvement. Graph courtesy UC Davis

Fortunately, we’ll never know how green and murky the lake might have become if we — and previous generations — had done nothing. If committed volunteers hadn’t formed the League to Save Lake Tahoe to protect the lake and preserve its legendary clarity back in 1957. If in 1969 California and Nevada hadn’t formed the Tahoe Regional Planning Agency (TRPA) to ensure better planning in the Basin.

It hasn’t been easy — or cheap. We’ve adopted onerous regulations. We’ve spent hundreds of millions of dollars of public money. And our work is not done.

These days, one of the biggest culprits is traffic. Cars kick up and produce pollution that adds those unwanted nutrients which feed the algae in the lake. That’s why TRPA quantifies and tracks car travel in the Tahoe Basin. They measure all the cars and all the traffic in terms of Vehicle Miles Traveled (VMT).

They even have a threshold for how much traffic, measured in VMT, a proposed project would generate in order to have a “significant” negative impact on the lake. If a new development project would generate 200 daily VMT, according to TRPA, that’s “significant.”

While TRPA uses that threshold to evaluate potential projects within the Basin, increasingly, the problem is development approved for just outside the Tahoe Basin — and outside TRPA jurisdiction — that pumps traffic into the lake’s watershed.

Alterra Mountain Company’s proposed extension of The Village at Squaw Valley, for example, would add more than 23,000 VMT to the Tahoe Basin — more than 100 times TRPA’s threshold for significance.

For generations, we’ve done so much to protect pristine Lake Tahoe from reckless projects within the Basin. We can’t let new development outside the basin render all that work meaningless.

Clearly, Tahoe deserves better.

There is, however, renewed hope in the latest readings. When scientists from the UC Davis Tahoe Environmental Research Center dropped their Secchi Disks in Tahoe last year, they lowered them a little deeper. The string got longer.

Tahoe’s clarity actually improved — by more than 10 feet.

Angelo Secchi is smiling upon us — and our lake. Abby can’t wait to get out on the water and measure the good news herself.

~ Tom Mooers is executive director of Sierra Watch