Workers vacuumed and soaked up pockets of oil along a 20- to 30-mile stretch of the flooded Yellowstone River in Montana Wednesday after as much as 1,000 barrels of oil spilled from a ruptured Exxon Mobil Corp. pipeline days earlier.

Floodwater carried oil into pastures and ditches along the banks of one of America’s most picturesque rivers, leaving vegetation and a handful of birds stained brown, said Jim Martin, regional administrator for the Environmental Protection Agency.

Cleanup crews, which include employees and contractors of Exxon Mobil, the Environmental Protection Agency, the U.S. Coast Guard and the state of Montana, laid pads to absorb oil and, near large pools, booms to collect and vacuum it, Mr. Martin said.

Cleanup crews worked to clear oil. To broaden its search for the oil, Exxon planned to test whether the river was safe to navigate with a jet-powered boat.

Oil is concentrated on a stretch of about 20 miles of river between Laurel, Mont., and Billings, Mont. However, federal officials say flights over the area showed indications of oil as far as 240 miles downstream of the rupture.

The river flows through Yellowstone National Park in Wyoming into Montana, past the 104,000 residents of Billings and toward the Missouri River in North Dakota, where officials hope oil won’t reach. Many of its banks traditionally have been pristine, teeming with wildlife and offering some of the world’s most famous fly fishing.