Geoscience Reference
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i t een submerged tree stumps that had escaped Harding's attention, some
measuring up to three and a half feet in diameter and seven feet tall. h e
core samples and radiocarbon dates from this much larger population of trees
rei ned and extended the boundaries of the mid-Holocene drought, mov-
ing its beginning to approximately 6,290 years ago and its ending to 4,840
years ago. h ese stumps, located deeper in the lake, showed that the lake
level had dropped by even more than Harding originally thought—by more
than twenty feet. Lindstrom has since located tree stumps in more places
around the shores of Lake Tahoe, including Emerald Bay, Rubicon Point,
and Baldwin Beach. Stumps of similar age have also been discovered in other
nearby Sierra lakes, including Donner, Independence, and Fallen Leaf.
h e lowered shoreline of Lake Tahoe in the late 1980s revealed other
interesting traces of the past. Archaeologists discovered bedrock mortars
containing grinding cups and milling slicks on the west shore, close to Tahoe
City, providing evidence that humans lived there during the mid-Holocene.
h ese artifacts and others from archaeological sites around the Tahoe Basin
coni rm that humans were able to sur vive in the region despite the prolonged
drought.
pyramid lake
More evidence of the mid-Holocene drought has been found in Pyramid
Lake, located in northwestern Nevada. h is lake is hydrologically connected
to Lake Tahoe by the Truckee River, the only outlet for Lake Tahoe. At er
leaving Lake Tahoe, the river l ows down the steep eastern l ank of the Sierra
Nevada, through Reno, and forty miles northeast into Pyramid Lake. Under
natural conditions, therefore, the size of Pyramid Lake is determined in large
part by the level of Lake Tahoe.
Pyramid Lake is a remnant of the ancient Lake Lahontan, which occupied
a large part of northern Nevada at the end of the last ice age (see i gure 18).
Pyramid is a relatively deep lake, at 890 feet, and has no outlet. h is makes
it ideal for paleoclimate research. Since 1970, Larry Benson, a geochemist
and hydrologist at the U.S. Geological Survey, has been studying what the
lake can tell us about the West's climatic history, including the relationship
between past climate change and human culture.
One of Benson's achievements has been the development of methods
for assessing past lake levels by measuring the oxygen isotopes in calcium
