Lewis was missing since about 8 p.m. Saturday, when a northbound Hallet boat carrying 10 people collided with a southbound Sleekcraft, which carried six people, according to officials.
Units from the Mohave County Sheriff's Office Division of Boating Safety photographed on September 2, 2018, the day after a boating crash on the Colorado River.
The crash happened in Moabi Regional Park, a popular water recreation area along the border about 290 miles east of Los Angeles.
The search resumed Sunday, but authorities said the toll of missing had risen to four at that point.
The cause of the crash is being investigated by Mohave County authorities, Sherwin said. The water ranged in depth from very shallow to 30 feet, he said.
The search included helicopters and divers.
Anita Mortensen, a spokeswoman for the Mohave County Sheriff's Office, says none of the boaters were wearing life vests when the collision occurred.
Search crews have been tirelessly scouring the river since the crash, with the sheriff's office saying it would continue to patrol "until all the unaccounted for passengers are located".