Denver ? A major spring snowstorm dumped more than a foot of snow across the Colorado-Wyoming state line on Thursday, canceling hundreds of flights, shutting down schools and making roads treacherous.

At least 15 people were treated at Cheyenne Regional Medical Center for injuries from three pileups involving about 50 vehicles on Interstate 25 just south of the state line, hospital spokeswoman Leslie Cook said.

The crashes led Colorado officials to close more than 40 miles of the road south of Cheyenne and a 45-mile stretch between Pueblo and Walsenburg. The highway is the main north-south thoroughfare in Colorado.

A 20-mile section of U.S. 50 between Pueblo and Penrose reopened Thursday evening.

Winds were gusting to nearly 40 mph in Denver, about 100 miles south of Cheyenne.