Number range CHAPTER© 2005 by Leo Poppoff

March 24, 2005

Basin Watch

by

Leo Poppoff

HOW MUCH SHORELINE HAVE WE LOST IN THE LAST SIXTY YEARS?

High waves, high winds, high lake levels, and erodible soils on three
sides of Lake Tahoe add up to loss of beaches. Well, that's not news.
Folks have been complaining about it since the first dam was built near
Tahoe City. Though we see signs of erosion on much of the shoreline, its
effect on the quality of Lake Tahoe's water has been a long-standing
question.

The latest and probably the last word on the subject, "Shorezone Erosion
at Lake Tahoe: Historical Aspects, Processes, and Stochastic Modeling,"
was published recently by Desert Research Institute scientist, Ken
Adams. He and his colleague, Tim Minor, studied shorezone changes by
analyzing aerial photographs that date from 1938. Since no one flew
balloons to take aerial photos of the lakeshore during the late 1800s,
only shorezone changes since 1938 can be tracked.

In addition, Adams and Minor determined what kinds of soils are present
along the

shoreline, and what kinds and amounts of nutrients they contain. They
inspected the shoreline and recorded signs of erosion. Then, they
calculated how much sediment and nutrient was washed into Lake Tahoe
during that sixty-year period.

The earliest set of photos (from 1938 to 1952) was from U.S, Forest
Service files. In 1995, TRPA made a series of photos around the lake.
And the USGS photographed the entire basin in 1992 and 1998. Each set of
photos was taken at a different scale, ranging from 1:8,000 to 1:20,000.
After Adams and Minor scanned and digitized the photos, errors in
measuring features on the images were about one meter, far exceeding
National Mapping Accuracy Standards.

The first challenge, according to Adams and Minor, was to define a
consistent and obvious shoreline feature, one that can be recognized on
multiple generations of aerial photographs of varying quality. They
chose the line between wet and dry sediment as the shoreline because it
approximates the mean high water line. That shore-water interface
fluctuates for a number of reasons: lake level, wave runup, seiches
(tsunamis) and human activity, as well as erosion and accretion.

During the sixty-year period, lake level was the most significant factor
affecting the position of the shoreline. At places, like the South
Shore, the shoreline can move some ten meters or more with minor changes
in lake level. Over the past 100 years, Adams and Minor note, the lake
level has varied from a historic high of 1899.29 meters in July 1907 to
a low of 1895.96 on November 30, 1992. The legal high water limit is
1898.65 meters. A meter is 3.3 feet.

In analyzing the imagery, Adams and Minor found four different
situations. The first was that part of the shoreline, such as the rocky
eastern shore, had not changed. The other three were beaches that had
eroded (such as along Homewood), accreted (as around the mouth of
Blackwood Creek) or accreted and eroded (as along the Edgewood Golf
Course).

For the latter three cases, Adams and Minor measured the local slopes of
the beaches and calculated where the shoreline would have been if the
lake levels of the earlier photos had been the same as those in the 1998
photos. Differences between the 1998 shorelines and the calculated
earlier shorelines were plotted and the changes were measured. If the
shorelines moved landward, there was obviously erosion over the
sixty-year period. If they moved lakeward, there was accretion. It all
sounds quite easy. But, to achieve the precision and reliability needed
for these results required a lot of painstaking work.

Adams and Minor concluded that, during the past 60 years, a total area
of 190,600 square meters of shorezone had eroded. They also found that a
total of 56,500 square meters had undergone accretion. The total mass of
sediment eroded into Lake Tahoe since 1938, according the DRI
scientists, is approximately 429,000 metric tons, or an average of 7150
metric tons per year. Some 33,000 metric tons were silt and clay, which
are composed of the finer particles that affect water clarity. On
average, 550 metric tons of silt and clay wash erode into the lake each
year from the shoreline. A metric ton is 2200 pounds.

Chemical analyses revealed that most of the eroded soils were richer in
phosphorus than nitrogen. Exceptions were Lake Forest, where nitrogen is
unusually high, a seep below a grassy area at Glenbrook and Third and
Blackwood Creeks. A total of 117 metric tons of phosphorus and 110
metric tons of nitrogen were introduced into Lake Tahoe by shoreline
erosion over the 60-year period, or an annual average of about two
metric tons of phosphorus and 1.8 metric tons of nitrogen.

Adams and Minor conclude that shorezone erosion contributes some 4% of
the phosphorus loading to Lake Tahoe, but less than 1% of the nitrogen.
They emphasize that these are percentages of the entire nutrient loading
to the lake from all sources. If estimates of other sources are scaled
back in the future, shorezone erosion will become more important.

Questions or comments? Send them to <basinwatch@sbcglobal.net>