Bridger Pipeline LLC was so sure its Poplar oil line was safely buried below the Yellowstone River that it planned to wait five years to recheck it. But last month, 3.5 years later, the Poplar wasn't eight feet under the river anymore. It was substantially exposed on the river bottom—and leaking more than 30,000 gallons of oil upstream from Glendive, Montana.

An ExxonMobil pipeline wasn't buried deeply enough for the Yellowstone River, either. High floodwaters in 2011 uncovered the Silvertip pipe, leaving it defenseless against the fast-moving current and traveling debris. It broke apart in July, and sent 63,000 gallons of oil into the river near Laurel, Montana.

Both companies underestimated the river's power and its penchant for scouring away the earth that's covering and protecting their pipelines. That miscalculation led to the Exxon Silvertip spill and it's likely to be declared a significant factor, at a minimum, in the Poplar spill.

Such misjudgments have potentially troubling implications nationwide, since pipelines carrying crude oil and petroleum products pass beneath rivers and other bodies of water in more than 18,000 places across America. Many of them are buried only a few feet below the water.

"There were a lot of people who wanted to think that the last pipeline spill in the Yellowstone River in 2011 was a freak accident that would never happen again. After this most recent spill, no one believes that anymore," said Scott Bosse, Northern Rockies director for American Rivers. "The truth is, there are probably hundreds of pipelines across the country that are at considerable risk of rupturing under our rivers."

While corrosion is the No. 1 cause of pipeline spills, a sizable number of pipelines at water crossings have ruptured or been endangered by river scour. Among them:

► The Poplar (Jan. 2015) and Silvertip (July 2011) pipeline failures on the Yellowstone River.

► More than 20 pipeline river crossings in Montana were found to be "dangerously close to exposure" during inspections of nearly 90 pipeline crossings in 2011, according to one report. Many of them have since been reburied significantly deeper. The Poplar pipeline was not among the crossings tagged as being close to exposure.

► Nearly half of the 55 oil and gas pipelines that cross the Missouri River were found to have sections buried 10 feet or less below the riverbed, according to the Wall Street Journal. A study by the U.S. Geological Survey, meanwhile, found that the Missouri riverbed had deepened by nine to 41 feet in 27 places because of severe scouring during the 2011 floods.

► An Enterprise Products Partners LLP pipeline that was uncovered by river scouring and ruptured in August 2011. The line spilled more than 28,350 gallons of a gasoline additive into the Missouri River in Iowa.

► A June 2012 spill in Alberta, Canada, where an oil pipeline owned by Plains Midstream Canada failed along the Red Deer River and released more than 122,000 gallons of light crude. Investigators concluded that the pipe was uncovered by scour during high flood waters and subjected to vibrations from the river flow that led a weld to fail.

► Three Enbridge Corp. crude oil pipelines crossing Minnesota's Tamarac River were exposed by floodwater erosion years ago, and were still exposed in mid-2014. None of the pipes had failed at that point, but one was being propped up by steel legs, according to an MPR News account.

Federal regulations aren't much help. The only rule that addresses pipe burial at major river crossings requires petroleum pipelines to be laid at least four feet below the riverbed at the time of construction. Once a pipeline's installed, there are no requirements regarding burial depth. There is no rule requiring exposed pipelines to be reburied, though a spill under those conditions would invite regulatory penalties for leaving the line exposed to hazards.

What's more, federal rules put the pipeline companies in charge of identifying all threats that could cause a spill in highly populated or environmentally sensitive areas, and the companies get wide latitude in deciding what to do about them, according to Rebecca Craven, program manager at the Pipeline Safety Trust, a nonprofit group that tracks pipeline risks and regulations.

"Operators with river crossings are supposed to figure out whether and to what extent scour is a risk," said Craven. "If it turns out that Bridger's line was inadequately protected like the Exxon pipeline was, it shows that we still have a problem with operators not doing adequate risk assessments."

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Exxon, for example, concluded that the Silvertip was not at risk during the 2011 floods because the pipeline wasn't damaged in past floods and hadn't recently been affected by scour. Federal regulators fined Exxon $1 million, calling Exxon's assumptions "not reasonable," and warning that the absence of damage in previous floods "does not mean future flooding could never cause a failure."

Indeed, the required four-foot minimum initial burial depth for pipelines can be completely eliminated by natural erosion over time or by a single flood event. Active free-flowing rivers can carve with enough ferocity to lower their riverbeds by 20 feet or shift the waterway onto an entirely new path, which can add new stresses to the pipeline or put the river over pipe that has less cover or lacks reinforcement or protective cement casings.

The hotly debated Keystone XL oil pipeline project would cross nearly 2,000 rivers, streams and reservoirs in Montana, South Dakota and Nebraska, according to one estimate. The route takes the pipe across the Missouri and Yellowstone rivers, where owner TransCanada has pledged to install the pipeline 35 feet below the riverbeds.

But conservationists, river advocates and pipeline opponents aren't reassured. They are especially worried about potential spills in rivers because they are often critical water sources for nearby communities. The iconic Yellowstone River, which has 39 fossil fuel pipelines that cross it or pass through its lateral migration zone, is also home to threatened and endangered wildlife, and a popular destination for fishing and rafting.

Kristi Ponozzo, public policy director at Montana's Department of Environmental Quality, said the state is anxious to put an end to oil releases into the Yellowstone. "We are looking at the situation and trying to figure out what actions could or should be taken to help prevent future spills," she said.

The Poplar Problem

The Poplar pipeline leaked on Jan. 17, sending more than 30,000 gallons of oil into the Yellowstone and contaminating the drinking water for residents of nearby Glendive, Montana. Oil recovery was difficult and dangerous because a layer of ice coated the river at the leak location and for several miles downstream.

Bridger Pipeline last checked the pipeline's burial depth in Sept. 2011 and planned to reassess the crossing in 2016. The 2011 survey showed the Poplar was buried at least eight feet below the river bottom of the river, the company said.

Sonar testing after the spill, however, showed that about 110 feet of the Poplar pipeline was completely uncovered along the bottom of the Yellowstone River. Up to 22 feet of the exposed section is unsupported because river scour created a one-foot deep gap between that part of the pipe and the river floor.

The cause of the spill is not yet known. The Poplar pipeline was built in the 1950s with pipe known to be susceptible to cracks and defects along its lengthwise seams, but the segment under the Yellowstone was replaced in 1967 with seamless pipe.

Stephen Holnbeck, a Helena, Mont.-based hydraulic engineer with the U.S. Geological Survey's Wyoming-Montana Water Science Center, said what's known about the spill so far points to scouring as a key factor.

"The thing probable scoured out sometime between 2011 and this last year's runoff, and it was sitting there exposed, and then there was some straw that broke the camel's back," Holnbeck said. "It could have scoured in 2012 and just been sitting there for the last three years."

Exposed pipe in a large river can be damaged by debris tumbling downstream, boat anchors, and other objects. In addition, long-term contact with the river can accelerate corrosion, and powerful currents can weaken welds or exacerbate defects. In the winter, ice can cause scouring and ice jams can damage unprotected pipelines.

"We were disheartened to learn that the line was exposed on the riverbed," Bridger spokesman Bill Salvin said after the sonar testing. The company is studying the accident and river dynamics, and will apply what it learns "across our system," he said.

The oil pipelines considered most vulnerable are those that are exposed or buried in relatively shallow trenches under active rivers that can flood and erode the riverbed. But each pipeline crossing is unique and constantly changing, and that means a shallow pipeline can be safe in a placid waterway and a deeply buried line can be in danger under a powerful river.

Stewart Rood, a professor who studies river science and dynamics at the University of Lethbridge in Alberta, said river crossings pose special risks for pipeline owners because stream characteristics are constantly changing and sediment can be removed and replaced in cycles.

"The Yellowstone's a big river, so eight feet doesn't sound like a lot to me," Rood said. "Because of the dynamic nature of a river, it is always eroding, always moving, and always eating away at the bed."

Call for New Federal Rules

Three years ago, after Exxon's Silvertip broke open, Montana's governor formed a pipeline safety council to scrutinize the petroleum pipelines that cross the state's key waterways. A separate group, led by the Yellowstone River Conservation District Council, used grant money to compile a pipeline river crossing database and produce a report that assigned risk levels to each of the major petroleum pipeline crossings along the Yellowstone.

Both of the state groups sought and received help and data from the federal Pipeline and Hazardous Materials Safety Administration (PHMSA), the primary regulator for U.S. pipelines carrying oil and other hazardous liquids. PHMSA conducted the nearly 90 river crossing inspections in Montana that revealed more than 20 vulnerable crossings.

Exxon, CHS, and Phillips 66 reburied pipelines at 12 river or creek crossings, using horizontal directional drilling to place the pipes substantially deeper. Several other pipeline crossings were strengthened with added cover, or other reinforcement.

Bridger's Poplar pipeline crossing was initially labeled a "moderate risk" for dangerous scouring by the state's risk assessment report, which relied on pipeline companies for some data. An updated version downgraded the site to "low risk," noting that the pipeline crossing had five feet of cover and that Bridger was monitoring it. It's unclear why the report cites five feet of covering, a shallower figure than the company's statement that the pipeline was at least eight feet below the riverbed at last check.

Both depth numbers exceed PHMSA's four-foot burial guidance for rivers that are more than 100 feet wide. Pipelines that cross smaller waterways need only be 30 inches below the river bottom at the time of construction.

"It's pretty apparent on the Yellowstone that the [federal] criteria for the trenched pipelines have failed us twice now," said Karin Boyd, a consultant and expert on fluvial geomorphology, or river processes, who worked on the Montana risk assessment report. "There was this much cover on the Bridger pipeline. People thought it was OK and it wasn't."

Bosse of American Rivers, said the federal rules aren't adequate.

For pipelines that cross rivers, he said, "safety standards need to be especially rigid...and pipelines need to be drilled very deep down under the bed of the river so they're not exposed to flooding."

But PHMSA, the agency that regulates pipelines, has issued a series of bulletins warning pipeline companies about the issue, and doesn't think new rules are needed.

After the Silvertip spill, Congress asked the agency to take stock of the river scouring problem and to decide whether PHMSA needed new regulations to address the issue.

In November 2013, Cynthia Quarterman, then-director of PHMSA, told Congress that depletion of cover played a role in 16 pipeline incidents between 1991 and October 2012, and amounted to just 0.3 percent of all hazardous liquids pipeline incidents in its database for that period.

PHMSA rules already require pipeline operators to assess and address all risks that could reduce the safety of the pipeline or cause it to fail, Quarterman said. Those risks include "flooding and lack of cover" in water crossings.

The Association of Oil Pipe Lines, a trade group that represents pipeline owners, backs the agency's conclusion.

"It's a concern, and we need to have a law on the books, and we do. And we need to have regs on the books, and we do," said AOPL spokesman John Stoody. "But it's such a rare occurrence overall, that it's something we have to think hard about, because we don't want to divert safety spending away from something else that's more frequent or has a bigger impact on human health and the environment."

Craven at the Pipeline Safety Trust is hopeful that, even without new rules, pipeline operators will be more vigilant about the threats at pipeline river crossings, and recognize that unexpected events can and do happen.

"Even in little creeks there's constant bedload movement, and given the right circumstances, you can get scouring under the creek," said Craven. "It happened to a little creek in Indiana. It had a flood one winter and it just changed course and all of the sudden, there's a pipeline there completely exposed."