Swimming in park waters is not recommended. At normal stage, the Green River flows at a relatively brisk 4.3 knots, about 5 miles per hour, which is too strong to swim against. There is no designated swimming area in the park and no lifeguards on duty. Do not be a victim of drowning! U.S. Coast Guard Approved personal flotation devices are required on the rivers in Mammoth Cave National Park. Drowning is the #1 cause of death in National Parks. Swimming in open, natural waters is very different from swimming in pools. Natural waters can change in depth unexpectedly, going from shallow to deep in just a few steps. Natural waters can also have swift currents and underwater hazards such as trees and rocks.