2909 17-Mile Drive © 2000 by Leo Poppoff

Pebble Beach, CA 93953

\(831\) 375-3302

July 13, 2000

Basin Watch

by

Leo Poppoff

WHAT DO THOSE MAGIC NUMBERS MEAN?

Three important numbers are often quoted when we discuss Tahoe's
environment. The numbers are one, ten and thirty. Like the DOW-Jones
Average or the NASDAQ, we recognize that these numbers are important.
But like the Dow and the NASDAQ, few of us really know what they mean
and experts can argue about interpretations.

Water clarity has improved this year. This one-year improvement has been
interpreted, by some folks, as showing that we're doing the right thing
in our attempts to stem the lake's decline. The
ten-years-to-save-the-lake number is too often interpreted to mean that
the lake will be green within a decade -- if we don't control watershed
erosion by then. And, the thirty-year number is interpreted as the time
it will take to see any signs of success of our efforts to stem the
lake's decline.

But, what do those numbers really mean? To find out, I went to their
source, the Tahoe Research Group (TRG) of UC-Davis. TRG's John Reuter
patiently explained it to me and I'll try to do justice to his efforts.

First, the one-year number. Reuter pointed out that this is actually the
second year of water clarity increases. And, he adds, that it's
promising \-- but not conclusive. It's certainly better than the
opposite trend. Still, there aren't enough historical data to confirm
water quality improvements.

The historical record, unfortunately, shows several two or three-year
spurts of seemingly improved water quality. In the mid-80s, there was a
period of five years of level or improving water clarity, and
unjustified euphoria. Yet, the long-term trend has been a relentless
loss of lake water clarity. So, don't pop the champagne corks yet.

Here's how Reuter explains the 10-year number. If the lake's water
quality continues to decline at the current rate, it'll be greenish in
about 30 years. TRG researchers base this on observations of other lakes
with clarity values are similar to those that Tahoe could have in 30
years - if nothing more is done.

Now, let's assume that it would take some twenty years to repair the
watershed. This includes the time it might take to construct erosion
control and water treatment projects, and for them to mature. For
example, new wetlands need time to develop proper soils, vegetation, and
bacteria before they can function adequately as water cleansers.
Vegetated slopes require a few years of care and feeding before they are
capable of stabilizing soils and stripping nutrients from surface flows.

And, most important perhaps, projects must be evaluated to see if they
are really doing the job. Based on those evaluations, revisions should
be made to correct deficiencies. Research could help decide how
watershed repair projects ought to be revised.

Evidence is mounting, for example, that our detention basins are not
very effective for dissolved phosphorus or fine-particle sediments.
Perhaps the basins should be larger to hold runoff longer. But the Tahoe
Basin is steep. There isn't much level space to build large detention
ponds or create new wetlands. Better evaluation and research could help
with this critical problem.

If we can do all these things in twenty years, and we have thirty years
before the lake is beyond the pale, that leaves us with ten years to get
going. The lake isn't expected to turn green in ten years, though it
will be moving in that direction. TRG researchers believe that we have
those ten years to get our act together \-- to do research, develop
plans, and start building more effective projects.

That prediction was made a couple of years ago. Fortunately, as Reuter
observes, activity has picked up. But we still don't have the rescue
operation organized.

According to TRG scientists, it would take the lake some six decades to
attain 95% of a new, healthier, equilibrium condition if there is a
decrease in its nutrient load. This estimate is based on sedimentation
rates of particles in the lake, and the depth of the lake. But it
doesn't account for the fate of dissolved nutrients. If we could
magically change the phosphorous load in the lake by a significant
amount -- say 50% -- we might see the new equilibrium in 2060, but we
ought to be able to detect some improvement in three or four decades.

Still, we can't magically decrease phosphorus loads that fast. So, what
that thirty-year number really means is that any real improvements we
make in the next decade or two won't be noticed for another few decades.
It also means that we need to look for other ways to judge how well
we're doing.

So, those numbers aren't really magic, and we needn't panic. But they're
good guides and we ought to pay attention.