Page 26
Field filled with basalt boulders dropped by the Lake Bonneville flood, south of Hagerman, (Oct. 1995). These are the "petrified watermelons."

Lake Bonneville and its Flood

Bonneville Basin
The Bonneville basin is a product of the Basin and Range faulting, which, starting about 17 million years ago, uplifted the Sierra Nevada Mountains on the west side of Nevada, the Wasatch Range in northeastern Utah, and the many ranges in between. Water that falls on these ranges does not reach the ocean, but flows to closed basins, of which the Bonneville basin is the easternmost.

The effect of climate change in the Lake Bonneville basin (the entire area that drains toward the modern Great Salt Lake, in western Utah, southernmost Idaho, and eastern Nevada) was to produce large lakes during the colder wetter glacial periods. During interglacial periods these lakes shrunk, and existed as saline water bodies like the present Great Salt Lake.

Calibration of Dating Methods
Geologists use several different methods to place ages on events related to Lake Bonneville and the Pleistocene in general. Radiocarbon years are based on the broadly applied method of dating organic material less than 30,000 years old with the naturally occurring isotope of carbon. Other methods of dating have been calibrated to tree rings and the growth rings of corals and are defined in "calibrated" years, which yield ages generally 10 to 20% older than radiocarbon years (cerling and others, 1994). Radiocarbon years are used in this book.

Click here for a larger view

(left) Quaternary lake levels in the Bonneville basin, after Currey and others (1984) and McCoy (1987).

(right) Map of Lake Bonneville and the path of its flood, after Malde (1968). Click on image for larger view.