Four people are missing after two recreational boats carrying 16 in total collided head-on Saturday night on a stretch of the Colorado River that marks the California border with Arizona, officials said.
An additional 10 people were hospitalized after the accident on a crowded Labor Day weekend.
The crash happened in Moabi Regional Park, a popular water recreation area along the border, about 290 miles east of Los Angeles.
The Mohave County Sheriff's Office said that all 16 people were ejected from the two boats on Saturday night and both vessels sank, leaving several people to be pulled from the water by other passing boats.
One woman who was picked up by a good Samaritan was found in critical condition, and after she was stabilized she was flown to the University Medical Center in Las Vegas. Nine others were taken to area hospitals by ambulance.
Search and rescue operations began Sunday morning with the help of helicopters, authorities said. The Mohave County Sheriff's Office said that it started dive and search operations in the area of the crash and warned the public to boat carefully if they enter that area of the river.
Eric Sherwin, a spokesman for the San Bernardino County Fire District, said the depth of the water on the river can range from shallow to 30 feet deep. Rescuers had called off the search for the missing until Sunday morning because it was too dangerous to put divers in the flowing water after dark. Some of those aboard the boats apparently were thrown into the water by the force of the collision or jumped, and were swept down the river, officials said.
"We had victims of this collision that were located 3 to 5 miles downstream from the original point of impact," Sherwin said.
Sonar images of one of the submerged boats did not appear to show anyone on board, he said. The other boat was badly damaged by the impact and later sank.
The Associated Press previously reported that only two were missing, and six people were taken to the hospital and six others only sustained minor injuries.