As modern rivers go, the Yellowstone is relatively unadulterated. It's the longest dam-free river in the U.S., rushing 692 miles from its headwaters in Wyoming's Absaroka Mountains through Yellowstone National Park and Montana's Paradise Valley and eastern plains, to its confluence with the Missouri. Cutthroat trout, vanished from much of their native range, still hold on there.

Before July 1, few people realized that oil coursed through pipes a few feet below the riverbed. Then, ExxonMobil's Silvertip pipeline failed, spewing an estimated 42,000 gallons of crude just upstream of Billings. It's too early to tell the extent of the damage, but birds and other wildlife might be impacted by oil accumulations in logjams, debris piles, riparian areas and quiet backwater channels. Tom Livers, deputy director for the Montana Department of Environmental Quality, says there's little precedent for evaluating oil's impact on riverside hay crops, many of which were soiled with crude spread by floodwaters. Fortunately, robust streamflow may have helped disperse the oil. "For a large, dynamic river like the Yellowstone, I think it should heal pretty quickly," says Scott Bosse, American Rivers' Northern Rockies director.

The accident was a stark reminder that oil spills aren't just for coastal folks. These inland messes are seldom Deepwater Horizon-style environmental disasters. But they're surprisingly common, and not at all innocuous. Last summer, a pipeline unleashed more than 800,000 gallons of tar sands crude into Michigan's Kalamazoo River; the polluted stretch still hasn't reopened for public use. A month earlier, some 33,000 gallons spilled into Red Butte Creek, a Jordan River tributary in Salt Lake City. Samples taken from sediment in the creek behind one home months later revealed known carcinogens above allowable levels. Just in the past few weeks, a small month-old leak was discovered on Montana's Blackfeet Reservation, and a BP pipeline in Alaska ruptured, leaking up to 4,200 gallons of crude, methanol and produced water onto sensitive tundra.

Why are these mishaps so common? Pumping pressurized oil and gas through pipes is inherently risky. But infrastructure is also aging, and pipeline safety regulations are often lax. Before 2001, internal inspections weren't even mandatory under federal law. Now, they're required only near so-called "high consequence areas," which include population centers and some navigable rivers, and cover about 44 percent of liquid and 7 percent of gas pipelines in the U.S.

Other laws are sufficiently vague to allow operators to more or less police themselves, says Carl Weimer, executive director of the watchdog group Pipeline Safety Trust. "(They) tell companies the end point they're shooting for," he says, "but leave it up to the companies to decide what they want to do." Pipelines are required to have leak-detection systems, but federal law doesn't specify what size leaks they must spot, or how quickly they must do so. And emergency shut-off valves, which can quell oil or gas leaks remotely, aren't required, though many companies use them voluntarily.

These regulatory weak spots help explain why oil seeped into the Kalamazoo for 12 hours before it was detected by nearby residents. In San Bruno, Calif., last year, it took nearly two hours to stop the flow of gas from an exploded pipe because a worker had to turn it off manually. Eight people were killed and 37 homes burned.

Safety advocates and environmentalists say that while regulators idle, the system may be growing more vulnerable. Crude from Canada's tar sands is increasingly transported to refineries in Montana, Wyoming and elsewhere, sometimes through pipelines built to handle significantly lighter and less corrosive conventional crude, which requires less pressure and heat to move. Higher corrosion rates and pressure could make such pipes more susceptible to spills. And the spills themselves pose new threats. Tar sands crude is mixed with liquid natural gas to become viscous enough to flow through pipes, making it more explosive. Due to its weight, a lot has settled to the bottom of the Kalamazoo River, complicating cleanup. Yet regulators have never examined whether existing rules are sufficiently protective to apply to this novel crude. Even brand-new pipelines built or proposed specifically to transport it still play by the old rules. Companies say that's not a problem; they don't want to see spills any more than the general public, and it's in their best interest to maintain pipeline integrity, given the bad PR and costs associated with failures. They have a point, says Weimer, "but really a federal regulator ought to verify this stuff."

The Silvertip pipeline regularly carried tar sands crude. But Exxon maintains it wasn't in the line when the spill occurred. The cause of the leak remains unknown. It took 56 minutes to plug, but was detected quickly.

The current theory is that high water washed away river-bottom sediment, exposing the pipe to floating debris that damaged it. If that's true, this spill's biggest lesson may be that river and aquifer protections are simply too weak, particularly in that pipelines can be buried relatively close to water sources. Better safeguards for rivers are being considered in the House, but haven't been included in a Senate bill to reauthorize the Pipeline Safety Act. Still, Weimer says both bills "include some pretty good stuff." The Senate's, for instance, would require more regular inspections, and directs the Department of Transportation to study leak-detection systems and report back to Congress on possible improvements.

"We're definitely going to be paying a lot more attention to pipeline design in so far as it affects rivers," says American Rivers' Bosse. "This Yellowstone River spill is a real wake-up call."