Firefighters said the river's water level was rising and it quickly reached the men's encampment, so they tried to swim across.
That's when the men ran into trouble. The river was about 18 inches deep and was moving very rapidly.
Someone spotted the men struggling in the water and called 911.
By the time paramedics arrived, the men had managed to get out, but they were in immediate need of attention because they were all suffering from hypothermia.
"Just a few minutes in the water, hypothermia will set in and then you lose the dexterity with your hands and then it's difficult to even pull yourself out," said Battalion Chief Craig Yoder of the Los Angeles City Fire Department.
Firefighters warned people not to get too close to the river. They said the banks are extremely slippery and it's very easy to fall in.
"Now is not the time to go out and do recreation in any of those areas nor is it authorized, and it's against the law at this point," said Battalion Chief Jack Wise.