California’s Tallest Bridge Has Nothing Underneath

Summary

The Foresthill Bridge in Auburn, California, stands as a striking and out-of-place monument to an ambitious but ultimately failed project: the Auburn Dam. Intended to provide flood control and water supply, the dam’s construction faced multiple challenges, including seismic risks and escalating costs. Despite the dam’s cancellation, the Foresthill Bridge, built as part of the project, remains a testament to a past era of large-scale public works. Its scale and significance, however, now represent both the promise and the pitfalls of American infrastructure development.

Takeaways

• 🌉 Foresthill Bridge, built in 1973, stands over 700 feet above the American River and appears oversized because it was designed to span the planned Auburn Dam reservoir.
• 🏞️ The Central Valley Project, launched in 1933, aimed to manage California's water by storing it in the north and distributing it to the drier south, providing flood control, irrigation, and hydropower.
• 🏗️ Folsom Dam, completed in 1955, offered limited flood protection and water storage, prompting the authorization of Auburn Dam in 1968 as a larger solution.
• 🪨 Geology at the Auburn Dam site was more complex than expected, with compressible rock and fissures requiring extensive foundation treatment to prevent structural failure.
• 🌊 Reservoir-induced seismicity became a major concern after the 1975 Oroville earthquake, showing that large reservoirs could trigger earthquakes in otherwise stable regions.
• 💥 The 1976 Teton Dam collapse further heightened safety concerns for Auburn Dam, as its thin-arch design would have been particularly vulnerable to seismic events.
• 🚧 While dam construction paused due to safety and environmental concerns, Foresthill Bridge was completed to ensure continued access in anticipation of the future reservoir.
• 💰 Rising costs, reduced benefits, environmental opposition, and improved flood control alternatives eventually stalled the Auburn Dam project, leaving it unbuilt.
• 🌱 Environmental and recreational impacts, such as flooding canyons, destroying ecosystems, and affecting whitewater rafting areas, contributed to public opposition.
• 🏛️ Foresthill Bridge remains today as a symbol of the ambitious Auburn Dam project and serves as a reminder of the caution needed in large-scale public works.
• 💧 Modern parallels, like PFAS water contamination and media bias awareness, highlight ongoing challenges in managing public resources, infrastructure, and information transparency.

Q & A

Why does the Foresthill Bridge seem out of place?

The Foresthill Bridge is an impressive structure built for a purpose that no longer aligns with its surroundings. It was constructed as part of the Auburn Dam project, which was intended to control the flow of the American River. However, the dam project was largely abandoned, leaving the bridge standing as an overbuilt and underused structure in a rural area.

What was the original purpose of building the Auburn Dam?

The Auburn Dam was authorized to provide flood control, hydropower, and water supply for California's Central Valley. It was designed to complement the Folsom Dam by regulating the flow of the North and Middle Forks of the American River, and to store water for agricultural irrigation and municipal use.

What factors led to the abandonment of the Auburn Dam project?

Several factors contributed to the abandonment of the Auburn Dam project. These included increasing construction costs, uncertainty about the dam's safety (especially after the Teton Dam collapse and concerns over seismic activity), environmental opposition, and improved flood control alternatives. Public and political support dwindled as costs mounted and the perceived benefits of the dam diminished.

How did the Oroville earthquake in 1975 affect the Auburn Dam project?

The 1975 Oroville earthquake raised concerns about the stability of large reservoirs in the region, especially regarding reservoir-induced seismicity, where the weight and pressure of water can trigger seismic events. The Auburn Dam, designed as a thin concrete arch, would have been particularly vulnerable to such earthquakes, leading to a reevaluation of the project’s safety.

What is 'reservoir-induced seismicity' and why was it a concern for Auburn Dam?

Reservoir-induced seismicity refers to the phenomenon where the weight and water pressure in a reservoir can alter the balance of forces along faults in the Earth’s crust, potentially triggering earthquakes. This was a major concern for Auburn Dam because the structure’s design relied heavily on the canyon walls for support, making it more vulnerable to seismic shifts.

What happened during the construction of the Auburn Dam site that complicated its progress?

During construction, engineers discovered that the geology of the site was more complex than anticipated. The rock underlying the dam’s proposed location contained compressible zones and numerous fissures, making it difficult to predict how the structure would behave under extreme pressure. This led to a significant foundation treatment program to stabilize the site, including pressure grouting to reinforce weak zones and reduce seepage.

Why was the Foresthill Bridge built before the Auburn Dam was completed?

The Foresthill Bridge was built in advance of the Auburn Dam to prevent access disruptions between Auburn and the Sierra foothills. With the river rerouted and a cofferdam in place, the existing bridge was at risk of being overtopped by floodwaters. The new bridge was designed to be high enough to span the future reservoir once it filled.

What makes Foresthill Bridge an impressive engineering feat?

Foresthill Bridge is one of the tallest bridges in the United States, standing over 700 feet (200 meters) above the American River. It was built with a striking steel cantilever design supported by two slender concrete piers, making it an impressive example of bridge engineering.

What was the outcome of the Auburn Dam project by the 1980s?

By the 1980s, the Auburn Dam project was essentially abandoned. New dam designs proposed in 1980 were deemed more feasible, but the project lost political and public support due to increasing costs, environmental concerns, and the availability of alternative flood control measures. The project never moved forward, and it was largely forgotten after the 1986 flood, which exposed issues at Folsom Dam.

What is the legacy of the Auburn Dam project in terms of American infrastructure?

The Auburn Dam project represents both the ambition and the caution of mid-20th-century American public works projects. It stands as a monument to the era of large-scale infrastructure planning, but also highlights the shifting priorities, public opposition, and environmental concerns that have since reshaped how such projects are approached. The Foresthill Bridge remains a symbolic reminder of this halted era of massive dam construction.