---
title: 'California''s Tallest Bridge Has Nothing Underneath'
source: 'https://youtube.com/watch?v=QOnQORqkHcM'
video_id: 'QOnQORqkHcM'
date: 2026-07-25
duration_sec: 1026
---

# California's Tallest Bridge Has Nothing Underneath

> Source: [California's Tallest Bridge Has Nothing Underneath](https://youtube.com/watch?v=QOnQORqkHcM)

## Summary

California's Foresthill Bridge, the fourth-tallest in the US, stands over 700 feet above the American River canyon, but it was built for a reservoir that never existed. This video explains the story behind the bridge—the failed Auburn Dam project, which was abandoned due to seismic concerns, cost overruns, and environmental opposition. The bridge remains as a monument to the end of an era of large-scale American dam building.

### Key Points

- **Foresthill Bridge Overview** [00:02] — Foresthill Bridge is over 700 feet (200 meters) high, the fourth-tallest bridge in the US. It sees only a few thousand vehicles per day, connecting small communities.
- **California's Central Valley Project** [01:11] — The Central Valley Project, started in 1933, aimed to store and distribute water across California. Folsom Dam was an early component, but it didn't provide enough flood protection or water storage.
- **Auburn Dam Authorization** [03:12] — Congress authorized Auburn Dam in 1968 to sit upstream of Folsom Dam. It would be the tallest dam in California. Construction began in the early 1970s.
- **Initial Construction Steps** [03:36] — Crews built a cofferdam and diversion tunnel to reroute the American River. They began foundation treatment, including pressure grouting to reinforce the rock.
- **Land Speculation and Development** [04:50] — Anticipation of the reservoir led to rising land values and development of lakefront communities around the future reservoir.
- **Oroville Earthquake Impact** [05:06] — A 5.9 earthquake near Oroville Dam in 1975 raised concerns about reservoir-induced seismicity, threatening the Auburn Dam's thin-arch design.
- **Engineering Report Halts Project** [06:33] — A 1976 report concluded that an earthquake like Oroville could cause catastrophic failure of Auburn Dam. The project was put on hold.
- **Foresthill Bridge Completed** [07:19] — The bridge was finished in 1973 to maintain access during construction. It was built high enough to span the future reservoir, but the dam was never built.
- **Project Cancellation** [08:34] — By 1980, costs had ballooned, public support waned, and alternative flood control measures made Auburn Dam unnecessary. The project was effectively dead.
- **River Restoration and Bridge Legacy** [13:09] — In 2007, the river was returned to its original channel. The bridge underwent seismic retrofits and remains a monument to the failed dam project.
- **Sponsor Segment: Ground News** [14:52] — The video is sponsored by Ground News, a platform that provides media bias analysis and context for news stories.

### Conclusion

The Foresthill Bridge stands as a striking monument to the Auburn Dam that was never built, symbolizing the end of an era of large-scale dam construction in the US. It serves as a reminder of the complex trade-offs in major infrastructure projects.

## Transcript

Foresthill Bridge soars across the valley  of the North Fork of the American River   just outside Auburn, California. At more than  700 feet or 200 meters above the canyon floor,  
it’s the fourth-tallest bridge in the United  States. When it opened in 1973, crowds cheered   for the impressive new structure. But if you take  a closer look, it doesn’t really make any sense.
This isn’t an interstate highway or even a major  thoroughfare. The road sees only a few thousand   vehicles a day, connecting Auburn, an exurb of  Sacramento with a population just shy of 14,000,  
to scattered rural communities and  recreation areas in the western   foothills of the Sierra Nevadas. And while  the American River does occasionally flood,   it doesn’t flood 700 feet. Before this, the  crossing was basically a low-water bridge.
A structure of this magnitude just looks out  of place. But it wasn’t just a boondoggle,   at least not at the outset. It  was built that way for a reason,   and the story behind it is not only pretty  wild, but it also sits at the hinge point  
of a major chapter in American infrastructure.  I’m Grady, and this is Practical Engineering.
California’s Central Valley is one of the world’s  great agricultural regions: over 400 miles long,   more than 50 miles wide, this remarkably fertile  area is nearly half the size of England. The  
city of Sacramento sits near its center, right  where the Sacramento and American Rivers meet. To manage and distribute water across this  enormous landscape, the federal government  
launched the Central Valley Project in 1933, a  sweeping effort by the U.S. Bureau of Reclamation   to store water in the wetter northern part of the  valley and distribute it to the drier south. In  
the process, the system would also generate  hydropower and reduce flood risk for growing   urban centers. I’m glossing over a lot here. The  history of California is steeped in water issues,  
and even just the Central Valley Project  is nearly a century of details. But,   critically, Folsom Dam was one of  the first big components of the plan. Built in 1955 on the American River, the  concrete gravity dam provided significant  
flood protection to the City of Sacramento.  However, it was constructed relatively early   in our understanding of basin-scale hydrology  and the uncertainty surrounding the frequency  
and magnitude of flooding over long periods  of time. It became clear pretty quickly that   Folsom Dam didn’t quite offer as much flood  protection as was originally promised. Plus,   because Folsom had to keep its flood  pool empty to handle potential inflows,  
its ability to store water for irrigation or  municipal supply purposes was somewhat limited. The answer to these problems, at least according  to the federal government, was Auburn Dam,  
authorized by Congress in 1968. The new structure  would sit upstream of Folsom and control the   variable flows of the North and Middle Forks of  the American River. It would be the tallest dam in  
California and one of the tallest in the country.  And work began in earnest in the early 1970s. One of the first steps in the process was  rerouting the American River. Crews built a large  
cofferdam and carved a diversion tunnel through  the canyon wall. With the water redirected,   they could begin drying out the bend in the river  where the huge new dam would eventually sit. Once the site was dried out, crews began exploring  the underlying geology more thoroughly. They  
drilled boreholes, excavated tunnels and shafts,  and surveyed the rock that would serve as the   dam’s foundation. The site’s geology turned out to  be more complex than expected. Some zones of rock  
were more compressible than others, which could  lead to dangerous stress concentrations in the   dam. And, there were a lot of joints and fissures  in the rock mass, making it more challenging to   predict how they would behave under extreme loads,  in addition to creating paths for water. So the  
next phase of the project was a major foundation  treatment program starting in 1974. This mainly   involved pressure grouting fractures to reinforce  weak zones against the enormous weight of the  
structure and to make the geology more watertight,  preventing seepage from flowing under the dam. anticipation for the reservoir was growing.  Around the future rim, land values soared,  
and developers rushed to stake claims. Lakefront  homes were planned. Entire communities emerged,   built on the promise of a shining new shoreline.  Then, in August 1975, a magnitude 5.9 earthquake  
struck near Oroville Dam, only about 50  miles or 80 kilometers away from the site. but it rattled confidence in the Auburn project.  The geology of the western Sierra Nevadas had  
long been considered stable. But the Oroville  earthquake introduced a troubling possibility:   that the loading and filling of large reservoirs  could trigger seismic events in the area.   This phenomenon, known as reservoir-induced  seismicity, is still not well understood even  
to this day. The pressure of water infiltrating  bedrock and the weight of a reservoir can   change the balance of forces along faults,  potentially triggering movement. You know,   when Oroville is full, that’s roughly 10 billion  pounds of force or 4 billion kilograms of mass.  
It’s a staggering amount. You can imagine  how that might affect the underlying geology. The Auburn Dam, as a thin concrete arch,  in contrast to the concrete gravity dam at   Folsom or the earthfill embankment at Oroville,  would be especially vulnerable to earthquakes.  
Thin-arch dams rely on the canyon walls to  resist the thrust of the structure. In fact,   I’ve made a video all about the topic you can  check out after this! If one side shifts even   a little during a quake, the results could be  catastrophic. In April 1976, a report by the  
Association of Engineering Geologists concluded  that an earthquake like the one at Oroville could   cause the proposed Auburn Dam to catastrophically  fail. It was back to the drawing board for the   project, even as the foundation grouting program  continued. And then the project was shaken again.
That same year, the newly completed Teton Dam  in Idaho collapsed during its first filling,   killing 11 people and causing billions in  damage. It had been built by the same agency,  
the Bureau of Reclamation. Concern continued  to mount about the safety of Auburn Dam,   which would have catastrophic consequences  for the thousands of Californians downstream   if it were to fail. It was all enough  to bring Auburn’s momentum to a halt.
While dam construction paused, one aspect  of the project had already been finished:   Foresthill Bridge. With a cofferdam on the  river and the diversion tunnel only sized   for smaller floods, there was a risk  of overtopping the existing bridge,  
cutting off access between Auburn and  the Sierra foothills. So, the Bureau of   Reclamation decided to get a head start on a  project that would eventually be inevitable:   a new bridge, permanent and high enough to  span the reservoir once it filled. If they  
were going to build a new bridge, they figured  they might as well build it right the first time. The result was a striking steel cantilever  bridge with two slender concrete piers soaring   skyward from the canyon floor. [Actually,  there was another bridge planned over the  
Middle Fork of the American River - the  Ruck-a-Chucky Bridge. It was a wild idea:   a curved cable-stayed bridge where all the cables  are anchored in the hillsides rather than tall   towers. But while that project was shelved,  Foresthill made it all the way through design  
and construction.] At the time of its opening  in 1973, it was the second-highest bridge in the   United States. But as time went on, it became  increasingly clear they had jumped the gun.
By 1980, engineers floated two new dam  designs that could withstand potential   earthquakes. Both would be shifted slightly  downstream from the original site. But by then,   the tide of public and government  support for the dam had turned.
Construction costs had ballooned, and Auburn Dam  was looking less feasible every day. As originally   proposed, the structure would be even larger than  the Hoover Dam size, but store less than 10% of  
Lake Mead’s volume. Meanwhile, upgrades to Folsom  Dam and improved levees around Sacramento offered   far cheaper ways to reduce the flood risk that was  the major impetus for the dam in the first place.  
New hydrologic data also suggested that earlier  flow estimates had been overly optimistic,   reducing its value for conservation. The  benefits of Auburn Dam were shrinking as the  
costs grew. It was turning into an incredibly  expensive solution in search of a problem. At the same time, environmental and advocacy  groups were gaining momentum. The project would  
flood canyons used for whitewater rafting  and kayaking. It would drown ecosystems,   inundate archaeological sites, and  destroy long segments of the wild   and scenic forks of the American River.  It became clearer and clearer that the  
ends simply couldn’t justify the means.  And yet, the idea never fully went away. In 1986, a massive flood hit the area. Water  backed up at the diversion tunnel at Auburn,  
overtopped the cofferdam, and caused it  to fail. Downstream levees were breached,   and much of Sacramento flooded. For a moment,  the momentum behind Auburn Dam and its promise   of flood protection returned. But, it later  became clear that the flood wasn’t entirely  
a natural disaster. The Bureau hadn’t followed  the operating guidelines at Folsom Dam, worsening   conditions downstream. And by then, grassroots  opposition, cost concerns, and shifting priorities  
had all but put the Auburn Dam project to bed.  Various proposals resurfaced over the years,   including the idea of a “dry dam” that would only  hold water during floods, but none gained much  
traction. With its many iterations and proposals,  the project became known as the dam that wouldn’t die. But in 2008, the state of California revoked  the Bureau’s water rights permit for the project,  
maybe not sealing its fate completely, but  at least burying it several feet deeper. This story really gets to the heart of the  challenge with large-scale public works   projects. No matter how you configure them,  there are big losers and big winners. There’s  
no doubt that a dam across the American River  upstream of Folsom could provide significant   benefits to the public: flood control, water  supply, hydropower, recreational opportunities,   or some combination of them all. But those  benefits have to be weighed against real costs:  
environmental damage, staggering capital  investment, long-term maintenance,   the inherent risk of catastrophic failure, and  the social toll of displacement and disruption. The mid-20th century was the  heyday of American dam building,  
an era driven by ambition and optimism, but also  by uncertainty. We didn’t have enough historical   data to fully understand river systems. We  couldn’t yet grasp the long-term consequences  
of altering them. And we couldn’t see into  the future to know what the true impacts of   these structures would be or what the cost of  keeping them in good shape might amount to. Since then, we have a lot more experience with  huge multi-purpose reservoirs. And it seems,  
in general, that the more we learn, the more the  answer to whether they’re worth it seems to be:   maybe not. And that maybe turns into a probably   New Melones Dam, completed by the Bureau of  Reclamation in 1979, not too far from Auburn,  
faced a lot of similar controversy  and pushback. Although the project   was eventually completed, the fight was  bitter, and its legacy so far is mixed.   The project is widely considered to be  the last great American dam. At least,  
great in size, if not in public sentiment. No  other reservoir of that scale has been built   in the U.S. since. And with the Auburn Dam project  mostly dead, it seems doubtful there ever will be.
The American River continued flowing  through the diversion tunnel until 2007,   when a new pump station and restoration project  returned the river to its original channel.   Kayakers can now navigate downstream, and even  have some new features at the pump station to  
choose from: the artificial rapids on the left  or the screen channel on the right. After more   than three decades, the river was back in its  place, tying a bow on a dam that was never built.  And yet, just a few miles upstream, the Foresthill  Bridge still stands, dramatic, overbuilt,  
and strangely out of sync with its surroundings.  And we’re still kind of stuck taking care of this   bridge, whose scale is so out of proportion with  its purpose. In the 2010s, the bridge underwent a   major seismic retrofit to improve its safety and  make future inspections easier. More recently,  
it was part of a nationwide program inspecting  bridges built with T-1 steel, an alloy that,   in some cases, has shown concerning cracking  at welds. The I-40 bridge crack in Memphis,  
which I covered in an earlier video, triggered  the effort. And there have been quite a few   defects found in bridges since then, so here’s  hoping that Foresthill doesn’t make the list.  It’s a cool structure in its own right. But  it stands for more than just an engineering  
achievement. Auburn Dam left a lot of scars, both  on the physical landscape and the political one.   But it also left this bridge that became more  than just an out-of-place oddity. In a sense,  
it’s become a monument to the end of an  era in US major public works projects,   and, hopefully, a tribute to the caution  and care that will shape the next one. 
era’s thought and care around water issues in  the US is “contaminants of emerging concern”:   stuff that can pollute drinking water that  hasn’t been historically regulated. Recently,  
the EPA rolled back limits on PFAS, the so-called  “forever chemicals,” in drinking water. The cost   of removing these compounds can be enormous,  and there are a lot of unknowns around their  
impact on human health. So, not everyone  agrees on what the limits should be. And   you can definitely get a feel for the controversy  if you read through the reporting on this story. More than 350 sources reported on the rollback,  with about 30 percent leaning left and 10 percent  
leaning right. Today’s sponsor, Ground News, makes  it easy to see them all in one place. But more   than that, it adds context to help you consider  any biases in the reporting. You can see ownership   and factuality ratings backed by independent  news monitoring organizations at a glance.  
All this is shown in a nice dashboard, with the  individual articles organized and linked below. If you compare the headlines, you can see the  different ways the story is framed. On the left,   the current administration is “undoing” or  “weaking” standards on “toxic” chemicals.  
On the right, you see softer language like  “easing” limits on “some” of the chemicals.   It starts to become obvious how news outlets can  slant stories in certain ways, depending on the   narrative they want to get across. In that  way, journalism has a lot of power over us,  
and Ground News hands some of that power back  to you. If you’d like a more transparent media   landscape, they’re offering a huge discount  right now at the link in the description:   40 percent off the Vantage subscription,  which includes unlimited access to all  
their features. That’s ground dot news  slash practicalengineering or just click   the link in the description. Thank you for  watching, and let me know what you think!