MACK, Colo. — An Amtrak train carrying more than 100 people derailed near the Utah-Colorado line after running into a pickup-sized boulder, authorities said Wednesday. Six people were injured, none seriously.

The California Zephyr, en route from Chicago to suburban Oakland, Calif., struck the 12-foot-by-12-foot boulder late Tuesday in a Colorado River canyon west of Grand Junction, Mesa County sheriff's spokeswoman Susan McBurney said.

Three engines and three to five cars went off the tracks, but none were reported in the river, McBurney said. A diesel spill was reported, but it was not considered a major problem.

An Amtrak spokesman, Clifford Black, said the accident injured four crew and two passengers "in the category of bumps and bruises." They were treated at a Grand Junction hospital and released, McBurney said.

Passengers were being taken to Salt Lake City by bus, Black said.

Western Colorado has been hit by a series of storms packing heavy snow at higher elevations and rain in the valleys, though when the rock ended up on the tracks was unclear.

"I know the conditions last night were very wet and rainy," McBurney said.

The tracks are owned by Union Pacific Railroad, whose spokesman, John Bromley, said finding out when the tracks were last checked for obstructions would be part of the investigation.

"We're all very thankful that there were no significant injuries," McBurney said. "Certainly it was a scary thing for the passengers."