[00:02] Foresthill Bridge soars across the valley&nbsp; of the North Fork of the American River&nbsp;&nbsp; just outside Auburn, California. At more than&nbsp; 700 feet or 200 meters above the canyon floor,&nbsp;&nbsp;
[00:14] it’s the fourth-tallest bridge in the United&nbsp; States. When it opened in 1973, crowds cheered&nbsp;&nbsp; for the impressive new structure. But if you take&nbsp; a closer look, it doesn’t really make any sense.
[00:27] This isn’t an interstate highway or even a major&nbsp; thoroughfare. The road sees only a few thousand&nbsp;&nbsp; vehicles a day, connecting Auburn, an exurb of&nbsp; Sacramento with a population just shy of 14,000,&nbsp;&nbsp;
[00:41] to scattered rural communities and&nbsp; recreation areas in the western&nbsp;&nbsp; foothills of the Sierra Nevadas. And while&nbsp; the American River does occasionally flood,&nbsp;&nbsp; it doesn’t flood 700 feet. Before this, the&nbsp; crossing was basically a low-water bridge.
[00:57] A structure of this magnitude just looks out&nbsp; of place. But it wasn’t just a boondoggle,&nbsp;&nbsp; at least not at the outset. It&nbsp; was built that way for a reason,&nbsp;&nbsp; and the story behind it is not only pretty&nbsp; wild, but it also sits at the hinge point&nbsp;&nbsp;
[01:11] of a major chapter in American infrastructure.&nbsp; I’m Grady, and this is Practical Engineering.
[01:25] California’s Central Valley is one of the world’s&nbsp; great agricultural regions: over 400 miles long,&nbsp;&nbsp; more than 50 miles wide, this remarkably fertile&nbsp; area is nearly half the size of England. The&nbsp;&nbsp;
[01:38] city of Sacramento sits near its center, right&nbsp; where the Sacramento and American Rivers meet. To manage and distribute water across this&nbsp; enormous landscape, the federal government&nbsp;&nbsp;
[01:50] launched the Central Valley Project in 1933, a&nbsp; sweeping effort by the U.S. Bureau of Reclamation&nbsp;&nbsp; to store water in the wetter northern part of the&nbsp; valley and distribute it to the drier south. In&nbsp;&nbsp;
[02:03] the process, the system would also generate&nbsp; hydropower and reduce flood risk for growing&nbsp;&nbsp; urban centers. I’m glossing over a lot here. The&nbsp; history of California is steeped in water issues,&nbsp;&nbsp;
[02:15] and even just the Central Valley Project&nbsp; is nearly a century of details. But,&nbsp;&nbsp; critically, Folsom Dam was one of&nbsp; the first big components of the plan. Built in 1955 on the American River, the&nbsp; concrete gravity dam provided significant&nbsp;&nbsp;
[02:32] flood protection to the City of Sacramento.&nbsp; However, it was constructed relatively early&nbsp;&nbsp; in our understanding of basin-scale hydrology&nbsp; and the uncertainty surrounding the frequency&nbsp;&nbsp;
[02:44] and magnitude of flooding over long periods&nbsp; of time. It became clear pretty quickly that&nbsp;&nbsp; Folsom Dam didn’t quite offer as much flood&nbsp; protection as was originally promised. Plus,&nbsp;&nbsp; because Folsom had to keep its flood&nbsp; pool empty to handle potential inflows,&nbsp;&nbsp;
[03:00] its ability to store water for irrigation or&nbsp; municipal supply purposes was somewhat limited. The answer to these problems, at least according&nbsp; to the federal government, was Auburn Dam,&nbsp;&nbsp;
[03:12] authorized by Congress in 1968. The new structure&nbsp; would sit upstream of Folsom and control the&nbsp;&nbsp; variable flows of the North and Middle Forks of&nbsp; the American River. It would be the tallest dam in&nbsp;&nbsp;
[03:24] California and one of the tallest in the country.&nbsp; And work began in earnest in the early 1970s. One of the first steps in the process was&nbsp; rerouting the American River. Crews built a large&nbsp;&nbsp;
[03:36] cofferdam and carved a diversion tunnel through&nbsp; the canyon wall. With the water redirected,&nbsp;&nbsp; they could begin drying out the bend in the river&nbsp; where the huge new dam would eventually sit. Once the site was dried out, crews began exploring&nbsp; the underlying geology more thoroughly. They&nbsp;&nbsp;
[03:53] drilled boreholes, excavated tunnels and shafts,&nbsp; and surveyed the rock that would serve as the&nbsp;&nbsp; dam’s foundation. The site’s geology turned out to&nbsp; be more complex than expected. Some zones of rock&nbsp;&nbsp;
[04:06] were more compressible than others, which could&nbsp; lead to dangerous stress concentrations in the&nbsp;&nbsp; dam. And, there were a lot of joints and fissures&nbsp; in the rock mass, making it more challenging to&nbsp;&nbsp; predict how they would behave under extreme loads,&nbsp; in addition to creating paths for water. So the&nbsp;&nbsp;
[04:23] next phase of the project was a major foundation&nbsp; treatment program starting in 1974. This mainly&nbsp;&nbsp; involved pressure grouting fractures to reinforce&nbsp; weak zones against the enormous weight of the&nbsp;&nbsp;
[04:36] structure and to make the geology more watertight,&nbsp; preventing seepage from flowing under the dam. anticipation for the reservoir was growing.&nbsp; Around the future rim, land values soared,&nbsp;&nbsp;
[04:50] and developers rushed to stake claims. Lakefront&nbsp; homes were planned. Entire communities emerged,&nbsp;&nbsp; built on the promise of a shining new shoreline.&nbsp; Then, in August 1975, a magnitude 5.9 earthquake&nbsp;&nbsp;
[05:06] struck near Oroville Dam, only about 50&nbsp; miles or 80 kilometers away from the site. but it rattled confidence in the Auburn project.&nbsp; The geology of the western Sierra Nevadas had&nbsp;&nbsp;
[05:23] long been considered stable. But the Oroville&nbsp; earthquake introduced a troubling possibility:&nbsp;&nbsp; that the loading and filling of large reservoirs&nbsp; could trigger seismic events in the area.&nbsp;&nbsp; This phenomenon, known as reservoir-induced&nbsp; seismicity, is still not well understood even&nbsp;&nbsp;
[05:41] to this day. The pressure of water infiltrating&nbsp; bedrock and the weight of a reservoir can&nbsp;&nbsp; change the balance of forces along faults,&nbsp; potentially triggering movement. You know,&nbsp;&nbsp; when Oroville is full, that’s roughly 10 billion&nbsp; pounds of force or 4 billion kilograms of mass.&nbsp;&nbsp;
[05:59] It’s a staggering amount. You can imagine&nbsp; how that might affect the underlying geology. The Auburn Dam, as a thin concrete arch,&nbsp; in contrast to the concrete gravity dam at&nbsp;&nbsp; Folsom or the earthfill embankment at Oroville,&nbsp; would be especially vulnerable to earthquakes.&nbsp;&nbsp;
[06:16] Thin-arch dams rely on the canyon walls to&nbsp; resist the thrust of the structure. In fact,&nbsp;&nbsp; I’ve made a video all about the topic you can&nbsp; check out after this! If one side shifts even&nbsp;&nbsp; a little during a quake, the results could be&nbsp; catastrophic. In April 1976, a report by the&nbsp;&nbsp;
[06:33] Association of Engineering Geologists concluded&nbsp; that an earthquake like the one at Oroville could&nbsp;&nbsp; cause the proposed Auburn Dam to catastrophically&nbsp; fail. It was back to the drawing board for the&nbsp;&nbsp; project, even as the foundation grouting program&nbsp; continued. And then the project was shaken again.
[06:51] That same year, the newly completed Teton Dam&nbsp; in Idaho collapsed during its first filling,&nbsp;&nbsp; killing 11 people and causing billions in&nbsp; damage. It had been built by the same agency,&nbsp;&nbsp;
[07:03] the Bureau of Reclamation. Concern continued&nbsp; to mount about the safety of Auburn Dam,&nbsp;&nbsp; which would have catastrophic consequences&nbsp; for the thousands of Californians downstream&nbsp;&nbsp; if it were to fail. It was all enough&nbsp; to bring Auburn’s momentum to a halt.
[07:19] While dam construction paused, one aspect&nbsp; of the project had already been finished:&nbsp;&nbsp; Foresthill Bridge. With a cofferdam on the&nbsp; river and the diversion tunnel only sized&nbsp;&nbsp; for smaller floods, there was a risk&nbsp; of overtopping the existing bridge,&nbsp;&nbsp;
[07:34] cutting off access between Auburn and&nbsp; the Sierra foothills. So, the Bureau of&nbsp;&nbsp; Reclamation decided to get a head start on a&nbsp; project that would eventually be inevitable:&nbsp;&nbsp; a new bridge, permanent and high enough to&nbsp; span the reservoir once it filled. If they&nbsp;&nbsp;
[07:50] were going to build a new bridge, they figured&nbsp; they might as well build it right the first time. The result was a striking steel cantilever&nbsp; bridge with two slender concrete piers soaring&nbsp;&nbsp; skyward from the canyon floor. [Actually,&nbsp; there was another bridge planned over the&nbsp;&nbsp;
[08:06] Middle Fork of the American River - the&nbsp; Ruck-a-Chucky Bridge. It was a wild idea:&nbsp;&nbsp; a curved cable-stayed bridge where all the cables&nbsp; are anchored in the hillsides rather than tall&nbsp;&nbsp; towers. But while that project was shelved,&nbsp; Foresthill made it all the way through design&nbsp;&nbsp;
[08:22] and construction.] At the time of its opening&nbsp; in 1973, it was the second-highest bridge in the&nbsp;&nbsp; United States. But as time went on, it became&nbsp; increasingly clear they had jumped the gun.
[08:34] By 1980, engineers floated two new dam&nbsp; designs that could withstand potential&nbsp;&nbsp; earthquakes. Both would be shifted slightly&nbsp; downstream from the original site. But by then,&nbsp;&nbsp; the tide of public and government&nbsp; support for the dam had turned.
[08:50] Construction costs had ballooned, and Auburn Dam&nbsp; was looking less feasible every day. As originally&nbsp;&nbsp; proposed, the structure would be even larger than&nbsp; the Hoover Dam size, but store less than 10% of&nbsp;&nbsp;
[09:04] Lake Mead’s volume. Meanwhile, upgrades to Folsom&nbsp; Dam and improved levees around Sacramento offered&nbsp;&nbsp; far cheaper ways to reduce the flood risk that was&nbsp; the major impetus for the dam in the first place.&nbsp;&nbsp;
[09:17] New hydrologic data also suggested that earlier&nbsp; flow estimates had been overly optimistic,&nbsp;&nbsp; reducing its value for conservation. The&nbsp; benefits of Auburn Dam were shrinking as the&nbsp;&nbsp;
[09:29] costs grew. It was turning into an incredibly&nbsp; expensive solution in search of a problem. At the same time, environmental and advocacy&nbsp; groups were gaining momentum. The project would&nbsp;&nbsp;
[09:41] flood canyons used for whitewater rafting&nbsp; and kayaking. It would drown ecosystems,&nbsp;&nbsp; inundate archaeological sites, and&nbsp; destroy long segments of the wild&nbsp;&nbsp; and scenic forks of the American River.&nbsp; It became clearer and clearer that the&nbsp;&nbsp;
[09:56] ends simply couldn’t justify the means.&nbsp; And yet, the idea never fully went away. In 1986, a massive flood hit the area. Water&nbsp; backed up at the diversion tunnel at Auburn,&nbsp;&nbsp;
[10:08] overtopped the cofferdam, and caused it&nbsp; to fail. Downstream levees were breached,&nbsp;&nbsp; and much of Sacramento flooded. For a moment,&nbsp; the momentum behind Auburn Dam and its promise&nbsp;&nbsp; of flood protection returned. But, it later&nbsp; became clear that the flood wasn’t entirely&nbsp;&nbsp;
[10:24] a natural disaster. The Bureau hadn’t followed&nbsp; the operating guidelines at Folsom Dam, worsening&nbsp;&nbsp; conditions downstream. And by then, grassroots&nbsp; opposition, cost concerns, and shifting priorities&nbsp;&nbsp;
[10:37] had all but put the Auburn Dam project to bed.&nbsp; Various proposals resurfaced over the years,&nbsp;&nbsp; including the idea of a “dry dam” that would only&nbsp; hold water during floods, but none gained much&nbsp;&nbsp;
[10:49] traction. With its many iterations and proposals,&nbsp; the project became known as the dam that wouldn’t&nbsp;die. But in 2008, the state of California revoked&nbsp; the Bureau’s water rights permit for the project,&nbsp;&nbsp;
[11:04] maybe not sealing its fate completely, but&nbsp; at least burying it several feet deeper. This story really gets to the heart of the&nbsp; challenge with large-scale public works&nbsp;&nbsp; projects. No matter how you configure them,&nbsp; there are big losers and big winners. There’s&nbsp;&nbsp;
[11:20] no doubt that a dam across the American River&nbsp; upstream of Folsom could provide significant&nbsp;&nbsp; benefits to the public: flood control, water&nbsp; supply, hydropower, recreational opportunities,&nbsp;&nbsp; or some combination of them all. But those&nbsp; benefits have to be weighed against real costs:&nbsp;&nbsp;
[11:38] environmental damage, staggering capital&nbsp; investment, long-term maintenance,&nbsp;&nbsp; the inherent risk of catastrophic failure, and&nbsp; the social toll of displacement and disruption. The mid-20th century was the&nbsp; heyday of American dam building,&nbsp;&nbsp;
[11:53] an era driven by ambition and optimism, but also&nbsp; by uncertainty. We didn’t have enough historical&nbsp;&nbsp; data to fully understand river systems. We&nbsp; couldn’t yet grasp the long-term consequences&nbsp;&nbsp;
[12:05] of altering them. And we couldn’t see into&nbsp; the future to know what the true impacts of&nbsp;&nbsp; these structures would be or what the cost of&nbsp; keeping them in good shape might amount to. Since then, we have a lot more experience with&nbsp; huge multi-purpose reservoirs. And it seems,&nbsp;&nbsp;
[12:22] in general, that the more we learn, the more the&nbsp; answer to whether they’re worth it seems to be:&nbsp;&nbsp; maybe not. And that maybe turns into a probably&nbsp;&nbsp; New Melones Dam, completed by the Bureau of&nbsp; Reclamation in 1979, not too far from Auburn,&nbsp;&nbsp;
[12:42] faced a lot of similar controversy&nbsp; and pushback. Although the project&nbsp;&nbsp; was eventually completed, the fight was&nbsp; bitter, and its legacy so far is mixed.&nbsp;&nbsp; The project is widely considered to be&nbsp; the last great American dam. At least,&nbsp;&nbsp;
[12:57] great in size, if not in public sentiment. No&nbsp; other reservoir of that scale has been built&nbsp;&nbsp; in the U.S. since. And with the Auburn Dam project&nbsp; mostly dead, it seems doubtful there ever will be.
[13:09] The American River continued flowing&nbsp; through the diversion tunnel until 2007,&nbsp;&nbsp; when a new pump station and restoration project&nbsp; returned the river to its original channel.&nbsp;&nbsp; Kayakers can now navigate downstream, and even&nbsp; have some new features at the pump station to&nbsp;&nbsp;
[13:25] choose from: the artificial rapids on the left&nbsp; or the screen channel on the right. After more&nbsp;&nbsp; than three decades, the river was back in its&nbsp; place, tying a bow on a dam that was never built.&nbsp; And yet, just a few miles upstream, the Foresthill&nbsp; Bridge still stands, dramatic, overbuilt,&nbsp;&nbsp;
[13:43] and strangely out of sync with its surroundings.&nbsp; And we’re still kind of stuck taking care of this&nbsp;&nbsp; bridge, whose scale is so out of proportion with&nbsp; its purpose. In the 2010s, the bridge underwent a&nbsp;&nbsp; major seismic retrofit to improve its safety and&nbsp; make future inspections easier. More recently,&nbsp;&nbsp;
[14:01] it was part of a nationwide program inspecting&nbsp; bridges built with T-1 steel, an alloy that,&nbsp;&nbsp; in some cases, has shown concerning cracking&nbsp; at welds. The I-40 bridge crack in Memphis,&nbsp;&nbsp;
[14:13] which I covered in an earlier video, triggered&nbsp; the effort. And there have been quite a few&nbsp;&nbsp; defects found in bridges since then, so here’s&nbsp; hoping that Foresthill doesn’t make the list.&nbsp; It’s a cool structure in its own right. But&nbsp; it stands for more than just an engineering&nbsp;&nbsp;
[14:28] achievement. Auburn Dam left a lot of scars, both&nbsp; on the physical landscape and the political one.&nbsp;&nbsp; But it also left this bridge that became more&nbsp; than just an out-of-place oddity. In a sense,&nbsp;&nbsp;
[14:40] it’s become a monument to the end of an&nbsp; era in US major public works projects,&nbsp;&nbsp; and, hopefully, a tribute to the caution&nbsp; and care that will shape the next one.&nbsp;
[14:52] era’s thought and care around water issues in&nbsp; the US is “contaminants of emerging concern”:&nbsp;&nbsp; stuff that can pollute drinking water that&nbsp; hasn’t been historically regulated. Recently,&nbsp;&nbsp;
[15:05] the EPA rolled back limits on PFAS, the so-called&nbsp; “forever chemicals,” in drinking water. The cost&nbsp;&nbsp; of removing these compounds can be enormous,&nbsp; and there are a lot of unknowns around their&nbsp;&nbsp;
[15:17] impact on human health. So, not everyone&nbsp; agrees on what the limits should be. And&nbsp;&nbsp; you can definitely get a feel for the controversy&nbsp; if you read through the reporting on this story. More than 350 sources reported on the rollback,&nbsp; with about 30 percent leaning left and 10 percent&nbsp;&nbsp;
[15:33] leaning right. Today’s sponsor, Ground News, makes&nbsp; it easy to see them all in one place. But more&nbsp;&nbsp; than that, it adds context to help you consider&nbsp; any biases in the reporting. You can see ownership&nbsp;&nbsp; and factuality ratings backed by independent&nbsp; news monitoring organizations at a glance.&nbsp;&nbsp;
[15:50] All this is shown in a nice dashboard, with the&nbsp; individual articles organized and linked below. If you compare the headlines, you can see the&nbsp; different ways the story is framed. On the left,&nbsp;&nbsp; the current administration is “undoing” or&nbsp; “weaking” standards on “toxic” chemicals.&nbsp;&nbsp;
[16:06] On the right, you see softer language like&nbsp; “easing” limits on “some” of the chemicals.&nbsp;&nbsp; It starts to become obvious how news outlets can&nbsp; slant stories in certain ways, depending on the&nbsp;&nbsp; narrative they want to get across. In that&nbsp; way, journalism has a lot of power over us,&nbsp;&nbsp;
[16:21] and Ground News hands some of that power back&nbsp; to you. If you’d like a more transparent media&nbsp;&nbsp; landscape, they’re offering a huge discount&nbsp; right now at the link in the description:&nbsp;&nbsp; 40 percent off the Vantage subscription,&nbsp; which includes unlimited access to all&nbsp;&nbsp;
[16:35] their features. That’s ground dot news&nbsp; slash practicalengineering or just click&nbsp;&nbsp; the link in the description. Thank you for&nbsp; watching, and let me know what you think!