© 2001 by Leo Poppoff

March 15, 2001

Basin Watch

by

Leo Poppoff

TAHOE'S AIR QUALITY THROUGH THE AGES

What was the quality of the Tahoe Basin air before the onset of
urbanization and smog? Well, there's precious little data to support an
answer to that question. Which leaves us free to speculate -- and that's
always a lot of fun.

For conditions during the Comstock Lumbering era in the Tahoe Basin, we
can get some ideas from Douglas Strong's "Tahoe: An Environmental
History." Strong cites excerpts from reports by travelers through the
basin during those days of high intensity logging, milling, slash
burning, and grazing. For example, "in 1889, the [Sacramento
Record-Union]{.underline} reported (that) witnesses fresh from the Lake
Tahoe region inform us that almost that entire section is a scene of
desolation on account of the destruction of fires . . . at the head of
Emerald Bay last week a fire was raging of many miles extent, destroying
the finest park of tree growth in all that region."

Another report told that a fire "started near Meyers in September 1898
swept up the mountains to the east." And, "when Charles H. Shinn, a U.S.
forest agent, visited the basin in 1902, he reported sighting many small
and smoldering fires, especially on the California side of the basin,
often four to ten in one day's travel." In addition, there were reports
of many fires set by sheep herders to burn off brush and encourage the
growth of grass.

We can guess that the skies were smoky during the summer logging season
during that period. And we can guess that skies really were clear much
of the time during the first half of the 1900s, while fire suppression
was the rule. But, what was it like before Comstock days?

The Washo people, it's believed, used fire to provide better visibility
through the forest and to encourage growth of valuable plants. So, we
might guess that it was smoky in the basin during the summer.

We can get a better idea from work on an airshed model, being developed
by UC-Davis atmospheric scientists. Steven Cliff and Thomas Cahill
describe the model and report preliminary results in their Chapter 3
(Volume I) of the "Lake Tahoe Watershed Assessment." They looked at how
prescribed burns and wildfires would affect air quality.

Burning 25 to 30 acres per day from May to October is equivalent to
burning the entire forest in the Tahoe Basin once every 40 years. This
is called a "fire return interval" of 40 years. And that's one of the
scenarios that Cliff and Cahill modeled. They hypothesized that this
might've been the rate of burning conducted by the Washo before European
types moved in and discouraged the practice.

According to Cliff and Cahill, ". . . visibility would be greater than
approximately 20 miles in the early morning within the basin. After
inversion breakup at about 11 A.M. each day during the burn period,
visibility would again increase to bring the north and south shores into
view. Historical burns generally occurred at times of excellent air
transport so the smoke was rapidly diluted and transported across and
out of the basin during the days. At night, however, the common
inversion would pull the smoke downslope and over the lake so that each
morning there would be smoke present at many areas of the lake. This
persisted until the late morning winds re-established the daytime
transport pattern."

Cliff and Cahill point out that this situation wouldn't have violated
TRPA's current visibility standards of at least 16 miles for at least 90
percent of the year. Still, according to Chapter 4 of the "Lake Tahoe
Watershed Assessment," by Patricia Manley and her associates, fire
return was more frequent in many areas of the basin. By studying fire
scars in tree rings on stumps, historic fire return intervals of 13 to
32 years were estimated for Jeffrey pine and mixed pine and white fir
forests on the east shore. Red fir forests had returns of 13 to16 years.
For some spots on the wetter west shore, returns were as high as 70
years.

Fire return intervals depend on precipitation and forest species. And,
since fire scars are made by only the most intense fires, it's likely
that fires were even more frequent. So, Tahoe skies might well have been
even smokier than the airshed model suggests.

Our intrepid environmental reporter of the day, Mark Twain, found the
air "very pure and fine, bracing and delicious . . . the same the angels
breathe." This was in August of 1861. If the Washo had been burning
vegetation with the frequency that we think they did, you might expect
that Twain would have made a different report. Perhaps those weren't
burn days, or the Washo had already been intimidated into not burning.
Whatever the case, Twain knocked over his campfire, set fire to the
forest, and fouled (at least temporarily) the air "the angels breathe."