A half-foot of rain in Pike County caused the Caddo and Little Missouri rivers to rise over 20 feet, flooding the campground at Albert Pike Park just outside of Mena.
State police reported a dozen people were killed and several others were stranded.
"We don't know who was in there last night," State Police spokesman Bill Sadler said. "This is a very wide area."
Sadler said officials were moving in a refrigerated truck to set up a temporary morgue. Searchers were working along the Little Missouri River in Montgomery and Pike counties and the National Guard dispatched helicopters to help in the rescue.
Rainfall totaled 7.6 inches at Glenwood, a weather reporting station about 20 miles from the scene, according to data from the National Weather Service at North Little Rock. A senior meteorologist reported that heavy rain fell throughout the region before moving into northern Arkansas late Friday morning.