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If you spend any time at all on the Yellowstone River, you are likely to be rewarded with stunning, sometimes unforgettable sights.

You may see bald eagles soaring overhead, or sitting in some lofty cottonwood tree observing your passage down the river. There are also pelicans and ospreys, foxes, otters and deer.

There are high sandstone cliffs and immense mounds of gray-black shale, gravel bars and stretches of sandy beach, red willows and churning runs of white-topped waves.

And then there is a common sight that some may find appalling and some may find strangely, incongruously beautiful — collections of classic American cars tethered to the banks with steel cables.

Here are the rusting hulks of Chevrolets, Fords, Buicks and other steel behemoths from the glory days of the American auto industry, placed on the riverbanks decades ago as a means of preventing erosion.

Their staying power is a testament to their effectiveness. Many of the antique autos are crammed with silt and driftwood and some of them have been there so long that fully mature trees have grown up through their engine compartments or trunks.

“You can’t use that kind of material anymore,” said Nicole McClain, coordinator of the Yellowstone River Conservation District Council. “It’s kind of an antiquity, really.”

The district council, in partnership with the U.S. Army Corps of Engineers and other agencies, has been working on a cumulative-effects study of the river that began in 2004 and has a target completion date of December 2015.

That study shows that while the cars may be the most eye-catching form of riprap — the blanket term for material used to prevent streambank erosion — they make up only a tiny portion of the riprap on the Yellowstone.

The inventory doesn’t yet cover the entire river, but it does quantify how much riprap is in place, and what materials are used, for the portion of the Yellowstone roughly between Laurel and Custer.

Through 2005, a little more than half a mile of Yellowstone River bank was lined with car bodies on that stretch of river.

Rock, the most common riprap material, was used to shore up about 17 miles of the same portion of the river, while concrete lined just under 13 miles of streambank.

Counting all forms of riprap, walls and weirs — flow deflectors built out into the river — the grand total came to about 40 miles on the stretch of river between Laurel and Custer, which is about 70 miles as the crow flies.

But keep in mind that the river does a lot of meandering, and that there are numerous islands and multiple channels, each channel with two banks.

There is no plan for getting rid of the car bodies, or for that matter any of the riprap, but current regulations prohibit the use of cars, as well as concrete containing reinforcement bar, for streambank stabilization.

Todd Tillinger, state program manager for the U.S. Army Corps of Engineers in Helena, said the corps does have the authority to make landowners remove materials that obstruct navigation or constitute a threat to river users, but it also realizes what a huge hardship that would be to most people.

“That’s something that’s a pretty serious thing to do,” he said. “It’s not something we do very frequently.”

In many cases, he said, the cars are protecting residences or public infrastructure and would have to be replaced with some other form of riprap if they were removed. It might be possible in some areas to simply cover the cars with several feet of soil and plant vegetation on top of them, he said.

Mother Nature has already done that in some cases, covering old car bodies with river silt during high water and then seeding the dirt with grass seed and other vegetation.

Tillinger said he wasn’t sure when cars were banned for use as riprap, but it probably dated from about 1975, when the Montana Natural Streambed and Land Preservation Act was established.

Karin Boyd, with Applied Geomorphology Inc. in Bozeman, who has worked on parts of the cumulative-effects study on the Yellowstone, said a “fond term” for the cars is “Detroit riprap.”

She said removing car bodies is “a difficult endeavor” that might release battery acid or other fluids into the river, or shake parts loose. There again, Mother Nature has already accomplished plenty.

On some stretches of the river, there might be 10 or 15 cars in a row lining a bank, and each car might have nothing left on it in front of the dashboard but the chassis.

At this low-water time of year, dry river bottom in the vicinity of these automobile graveyards is likely to be strewn with miscellaneous chunks of metal sticking up through the mud and gravel.

The problem is hardly unique to the Yellowstone, or to Montana. And here, as elsewhere, the concern isn’t so much with what lines the banks as with what happens to the material over time.

“It’s always been a concern,” Boyd said. “Whenever anything is put on the banks, it ends up in the river.”

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I cover the city of Billings.