BOZEMAN, Mont.—What happened under the churning waters of the Yellowstone River near Laurel, Mont., on July 1, won't be clear until the river subsides this coming fall, but the working theory is that the flood-stage river scoured down to a buried Exxon oil pipeline, busting open the pipe with a barrage of boulders and logs.

The Yellowstone River is the longest free-flowing river in the lower 48 states, meaning it has no dams along its length and thus scours its bed more. Two toads, a garter snake, and a warbler have received treatment so far from the spill. A few more oiled birds have been sighted needing treatment. No one knows the exact amount of oil that poured into the river before the company shut the pipeline off—initial estimates landed on 1,000 barrels of oil. Last year's Deepwater Horizon oil spill in the Gulf of Mexico, by contrast, released millions of barrels.