Forest Hill Bridge and Auburn Dam: The 700-Foot Crossing That Shaped California’s Public Works Era

The Forest Hill Bridge towers over the North Fork of the American River near Auburn, California, at more than 700 feet, making it the fourth-tallest bridge in the United States. This video examines not only the bridge’s dramatic scale but also the Auburn Dam saga, a bold but controversial plan tied to the Central Valley Project. It traces the engineering challenges, seismic concerns, political opposition, and the long shadow these projects cast on public works decisions in the American West. It also reflects on how a single crossing ended up out of place yet enduring, while the dam project it was meant to support fizzled in the face of cost, risk, and environmental questions.

Introduction: A Monument in a Valley

Practical Engineering presents Forest Hill Bridge as a dramatic crossing over the valley of the North Fork of the American River, just outside Auburn, California. At over 700 feet above the canyon floor, it was once the second tallest bridge in the United States and remains a striking symbol of mid-century engineering ambition. Yet the surrounding context makes the bridge feel incongruous with its setting and use, since the road it carries connects a small cluster of communities rather than a major interstate. This section sets up the paradox at the heart of the Auburn project: a high, monumental crossing built during a boom in dam construction that never fully realized its supposed flood control and water storage promises.

Background: Water, Dams, and a Valley-Wide Plan

The video situates Forest Hill Bridge within California's grand Central Valley Project, a 1933 federal effort to store water in the north and distribute it to the drier south, while also generating hydropower and reducing flood risk. Folsom Dam, completed in 1955 on the American River, offered some flood protection but left uncertainty about basin-scale hydrology and reservoir storage. Auburn Dam, authorized in 1968, would sit upstream and offer more control over the North and Middle Forks. Construction began in the early 1970s, and the new crossing at Forest Hill was deemed a necessary precaution to ensure access during reservoir filling and floods. This section introduces the big public works idea that would drive development for decades, setting the stage for the engineering and political drama to follow.

Site Investigations: Rock, Joints, and Foundation Treatment

As the Auburn site was prepared, diversions allowed the river to be dried out and boreholes, tunnels, and rock surveys were conducted. Engineers found a more complex geology than anticipated, with zones of rock that could deform under load and joints that could become pathways for seepage. The response was a major foundation treatment program beginning in 1974, focused on pressure grouting to reinforce weak zones and tighten the rock mass to withstand the dam’s enormous weight. This sequence highlights the core engineering challenge of large dam-building: predicting and controlling geotechnical behavior in a canyon setting, under extreme loads and long time scales.

Seismic Risk and Reservoir-Induced Seismicity

The project’s prospects were unsettled by a major earthquake near Oroville in 1975, which rattled confidence that a new dam could be built safely upstream of Folsom. The video explains reservoir-induced seismicity as the phenomenon by which filling large reservoirs alters rock stress and can trigger fault movement. The Oroville event underscored the risk that a thin arch dam like Auburn Dam would face, given its reliance on canyon walls for thrust resistance. In 1976, a report warned that a quake could cause catastrophic failure in the Auburn design. The same year, a nearby dam, the Teton Dam, failed during initial filling, underscoring broader concerns about the Bureau of Reclamation’s handling of multi-dam, basin-scale projects.

From Ambition to Reality: Auburn Dam’s Slow Decline

Despite early strides, the Auburn project faced rising costs and shifting priorities. By 1980, the engineering community proposed new dam designs downstream from the original site, but public support waned as evidence grew that the dam’s benefits might not justify its astronomical price tag. Environmental groups highlighted the loss of whitewater rafting, ecosystems, and archaeological sites, while hydrologic data suggested lower-than-anticipated benefits. The video traces how the project’s momentum faded, even as some components like Forest Hill Bridge were completed. By 1986, a large flood rediscovered the reservoir’s potential, but the Bureau’s own operating guidelines and downstream risk concerns undermined Auburn Dam’s viability. In 2008, California revoked the Bureau’s water rights, effectively sealing Auburn Dam’s fate, while downstream river restoration and pump station projects attempted to return the river to its natural course. The result is a cautionary tale about the costs, risks, and political contest surrounding mega-dam projects.

The Forest Hill Bridge: A Standalone Engineering Icon

While Auburn Dam never fulfilled its grand promises, Forest Hill Bridge persisted as a bold, out-of-scale crossing with two slender concrete piers rising from the canyon floor. The bridge remains a dramatic symbol of an era when multi-purpose dam projects dominated US infrastructure policy. The video notes its later seismic retrofit and ongoing inspections of T1 steel welds, connecting it to broader concerns about aging bridges and the integrity of critical infrastructure. The Forest Hill Bridge thus stands as a monument to both the ingenuity of mid-century engineering and the complex legacy of large public works that shaped California’s landscape and policy for generations.

Lessons for Public Works: Costs, Benefits, and the Public Interest

Toward the end, the video reflects on the mid-twentieth century heyday of dam construction, where ambition often outpaced knowledge. It argues that the more we learn, the more it seems that many of the best sites are already used, and that large, multi-purpose reservoirs carry significant environmental, social, and financial costs. The Auburn story demonstrates that even highly visible engineering feats can become symbols of a failed or transformed policy paradigm, while still leaving behind enduring infrastructure such as Forest Hill Bridge. The video closes with a broader meditation on how infrastructure decisions should balance flood control, water supply, and power with ecological preservation, safety, and long-term maintenance costs.