A Look Back in History: Bedrock Contour Map of Hetch Hetchy Dam Site

The original O’Shaughnessy Dam was 226 feet above riverbed. It was raised from its original height in 1938. Today the O’Shaughnessy Dam is 312 feet above riverbed and 430 feet above bedrock.

Below is a bedrock contour map developed for the dam. This illustration is taken from Contract No. 61, “For the Construction of Hetch Hetchy Dam and Appurtenant Works.” The date on this document is difficult to decipher. The date on the copy of the contract proposal is May 1919.

Bedrock Contour Map of Hetch Hetchy Dam (O’Shaughnessy Dam).

Actual groundbreaking on O’Shaughnessy Dam was on August 1, 1919. It was that date when San Francisco’s Utah Construction Company began clearing the trees in Hetch Hetchy Valley. So, the first step in preparing for construction was to ready the valley to become a reservoir.

Subsequently, the riverbed on the site of the future dam was excavated over 100 ft before hitting the granite bedrock. A retaining wall was poured on the upstream side to prevent water seepage into the foundation hole, and the granite was scoured and artificially roughened to prepare for receiving concrete.

A total of 398,516 cubic yards of concrete was poured to form a dam standing 226 feet above the riverbed and 344 feet above the foundation that rested on bedrock.