Opponents of the Bulgarian-born Christo's proposed "Over the River" project sued the U.S. Bureau of Land Management in Denver, saying the agency violated federal law and its own policies when it gave final approval to the project last fall.

The flamboyant Christo, known for his massive outdoor artworks, including erecting 7,500 fabric gates in New York City's Central Park, plans to hang 5.9 miles (9.5 kms) of "silvery, translucent" panels intermittently along the river between Canon City and Salida, Colorado, according to his web site.

But opponents seeking to halt the project argued in their lawsuit that allowing the drilling of some 9,000 bores to anchor 925 fabric panels would adversely impact wildlife, cause traffic tie-ups and disrupt recreational activities.

"Installing the anchor bolts and other hardware requires large drilling equipment normally used for mining and heavy construction activities. The art project is, in fact, more similar to mineral resource extraction and development than recreation," the suit said.

Bureau of Land Management spokeswoman Tina Brown said the agency was aware of the lawsuit, filed by Denver University law students on behalf of nonprofit Rags Over the Arkansas River Inc. ROAR.L. Brown said the bureau does not comment on pending litigation.

Christo is not a defendant in the lawsuit. But his lawyer, Steve Coffin, said the project went through a comprehensive and thorough review by federal land managers.

"We're confident that it will withstand legal scrutiny," Coffin said.

ROAR spokeswoman Joan Anzelmo characterized the case as a classic David versus Goliath battle.

"ROAR is aiming its slingshot directly at the Goliath Over the River project and at the Bureau of Land Management that gave the go-ahead despite its federal stewardship responsibilities for public lands, water and wildlife," she said in a statement.

Of particular concern to ROAR is the potential impact to a herd of bighorn sheep, Colorado's state mammal.

"The (project) will increase the bighorn sheep's susceptibility to disease, disperse sheep from their primary habitat along the Arkansas River, and will likely increase the mortality rate in young lambs," the lawsuit said.

The project, which has been in the works for more than a decade, still needs the green light from local jurisdictions before construction gets underway. Christo's plans call for it to be unveiled in August 2014 and stay up for a two-week period.

The river project is not Christo's first foray in Colorado. In 1972, his "Valley Curtain" project consisting of a fabric curtain suspended across the Rifle Gap in western Colorado was shredded by high winds hours after it was unveiled.

(Editing by Dan Whitcomb and Cynthia Johnston)

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