1980 | Diversion Tunnel, Auburn Dam Site
Water issues were the first order of business in the new California legislature in 1850. Then in 1862, newly-elected governor Leland Stanford was rowed to his inauguration, since the capital city of Sacramento in the Great Central Valley was under six feet of water.
Flood control has long been a concern here. Hydrologists and engineers will all tell you that in California, ‘’there is no such thing a normal winter.” The dramatic increase in population in the 20th century brought more concerns, since seventy-five percent of the water supply is in the north, and the majority of the population settled and grew in the south.
The North Fork of the American River produces prodigious flows during rains and rapid Sierra snowmelt conditions. Considerable planning went into finding a damsite to optimize flood control to support and protect downstream dams.
Proposed in the 1950’s, work began in 1968, but by 1980 the Auburn Dam construction site had become dormant, a kind of geological museum; the deep excavation cuts cleanly through a previously unmapped fault. The excavations had been made for abutments of a concrete thin-arch dam, but an earthquake in 1975 raised public concern about the dam design and construction was halted. Cost questions followed, then environmental concerns. But core-sampling continued, and a huge warehouse nearby catalogued the specimens, which provided a unique highly-detailed study of the entire site.
If a dam is ever built there, the decision will be made politically, and the plan may be for an earthfill or a concrete gravity structure instead. When I asked a thoughtful Bureau of Reclamation official when he thought the Auburn project might get the go-ahead, he told me, “Right after the next flood.”
In this picture, at the lower right is the discharge from the diversion tunnel, which was to carry the full flow of the river during dam construction. A flood in 1986 overwhelmed the tunnel’s capacity and eroded the cofferdam, which was designed with a flow-metering wash-away section. This broke through quickly, and the huge flow rammed down to Folsom Dam and on to the distant Delta levees. These barely held and were afterwards re-rated for a much lower flood threshold.