Vermilion Valley Dam is a large earthfill dam located in the Sierra Nevada mountains east of Fresno, California. Completed in 1954, the 165-foot high, 4,234-foot long dam, situated in a glacially carved valley containing glacial till and moraine deposits has had a history of seepage issues. During the summer of 2007, seepage was observed at the toe of the dam during high reservoir conditions. The reservoir was drawn down and an engineered subsurface filter system was constructed to intercept the predicted seepage. After several years of draught, 2010 proved to be a wet year and the reservoir levels were back to where they had been in 2007. Seepage was observed about 30 feet downstream of where the 2007 seepage was seen. After constructing a sandbag chimney to confine water, provide some head to resist soil movement, and allow quantitative monitoring of the seepage levels, Willowstick was contracted to perform a geophysical investigation of seepage through the dam. The results of their investigation are presented in this paper, as well as proposed upgrades to the filter system at Vermilion Valley Dam.
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