© 2003 by Leo Poppoff

November 28, 2003

Basin Watch

by

Leo Poppoff

TAHOE'S SHORELINE IS SLOWLY RECEDING

It's not really news that Lake Tahoe's shoreline is eroding. It's been
an issue for something like 130 years, ever since some early residents
of the Tahoe Basin and the Truckee Meadows (and maybe a couple more from
San Francisco) got the bright idea of damming the lake's outlet. Water
was impounded to flume logs down the Truckee River, to generate
electrical power and to provide extra water for downstream farmers and
city folk. It also raised the lake level and eroded the shoreline.

Like many Tahoe issues, this one has been rife with heat and opinion,
but there hasn't been much light. At various times, shoreline erosion
has been targeted as a serious source of pollution to the lake. At other
times, it's declared to be an insignificant source. Of course, it isn't
easy to find out just how big -- or little -- a problem shoreline
erosion might be.

Dr. Ken Adams, Assistant Research Professor at the Desert Research
Institute, has been gathering the kind of information that could shed
light on, and reduce the heat of, this issue. His approach has been to
find historic aerial photographs of the Lake Tahoe shoreline and
manipulate them so they can be overlaid on recent photos. This is a very
sophisticated, technical process that I won't even attempt to describe.
However, it's explained in a paper by Kenneth Adams and Timothy Minor,
entitled "Historic Shoreline Change at Lake Tahoe From 1938 to 1998 and
its Impact on Sediment and Nutrient Loading." It was published in the
Journal of Coastal Research, Volume 18, No. 4, 2002.

By studying changes in photographic features over the years, Adams is
able to tell where the shoreline has retreated, where it has advanced,
and where nothing much has happened. The oldest photos date from 1938,
which, Adams notes, are about the time aerial photography got started.
Other photos date from 1939, 1940, 1952, 1963, 1992, and 1995. The most
recent were taken in 1998. According to Adams and Minor, photos
from1940, 1952 and 1998 were especially useful because Tahoe's lake
level was almost identical when each set was taken.

Shorelines are eroded, Adams explains, by waves that overrun established
beaches and attack backshore escarpments. This happens during big storms
when Lake Tahoe is full. The researchers' analyses show that between
1938 and 1998, twenty-two areas of shoreline had lost a total of 190,600
square meters (about 47 acres). During that same period, twenty areas
had added a total of 56,500 square meters (about 14 acres).

Adams and Minor determined the "along shore" and "cross shore" distances
of eroded beaches by comparing 1939 shorelines to 1998 shorelines and
the thicknesses of eroded areas from detailed topographic maps. With
this, they were able to calculate the volume of soil that was eroded. It
amounts to 286,000 cubic meters. Typically, the eroded areas lost soil
layers one to two meters thick. And, according to Adams and Minor, up to
six meters of soil has been eroded in some spots. They also checked on
the types of soils eroded, and determined an average density.

With this data, researchers calculated that some 429,000 metric tons (a
metric ton is 2,200 pounds) of sediment had eroded into the lake over a
60-year period. This is an average of 7,150 metric tons per year. Adams
and Minor point out that this is the second largest source of sediment
to the lake. The largest is the amount that's transported into the lake
by streams, and that averages some 11,300 metric tons per year.

As soil erodes into the lake, it carries nutrients. The DRI researchers
sampled soil around the lake shore to determine the nutrient content of
different soil types. They estimate that during the 60-year period, some
117 metric tons of phosphorus and 110 metric tons of nitrogen washed
into the lake along with eroded soil. On average, some two metric tons
of phosphorus and 1.8 metric tons of nitrogen were introduced to the
lake. These are small fractions of the current annual loading of
nutrients. They represent 4 percent of the total phosphorus and less
than 1 percent of the total nitrogen.

Shoreline erosion had been thought to be an insignificant problem
because of the low nutrient loading. But recently, Tahoe researchers
have learned that fine soil particles are contributing significantly to
the loss of Tahoe's water clarity. So, shoreline erosion might be an
important source of pollutants after all.

Still, Adams cautions, the shoreline probably isn't eroding at the
60-year average rate. Much of the shoreline in developed areas is now
armored. Revetments have been built to protect buildings and roads
around the shoreline. This reduces erosion and subsequent introductions
of sediments and nutrients into the lake.

Can we reduce sediment loads by lowering the elevation of the lake in
winter? Adams reminds us that water impounded by the Tahoe City dam is
very valuable and protected by court orders. The chances of reducing
erosion that way are nil. Do revetments cause increased erosion of
neighboring parcels? Adams says that there is very little evidence of
that.

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