Anthony Alexander Johnson, 18, was swept away in the river's current after he and five friends jumped in at Kalamazoo's Merrill Park Saturday, the day chosen by Harold Camping in his highly publicized doomsday prophecy, the Kalamazoo Gazette reported.
Jordan Skinner-Knapp, 15, said he and his friends told Johnson not to jump because he couldn't swim well but Johnson jumped anyway.
Jordan said he tried to grab his friend but the current separated them and Johnson kept going downstream.
"I tried to save him," Jordan said.
Emergency rescue workers searched the river from the park to about a mile west until the light became too low, Kalamazoo County Sheriff Richard Fuller said.
People don't often swim in the river, Knapp said, and a sign says swimming is not allowed.
"It's very strong," Knapp said. "There's lots of undertow."