Use your senses—at least the three that can save your life, advises David Black, an American Canyoneering Association–certified guide. Smelling new moisture from rain can tip you off that it's time to get to higher ground. If you're in a naturally wet canyon, look for changes in water level and clarity caused by debris mixing with flood waters. Finally, listen for rapids and grinding boulders: "If it's a big water event," Black says, "it will sound like a freight train is coming." If anything seems off, start climbing. You may have minutes, or only seconds. Find the widest portion of the canyon, where the flow is less intense, and hold on to something solid. If you find yourself swimming, take a defensive position—float feet downstream and on your back. Of course, prevention is the best medicine, so get a detailed forecast before heading into any deep, narrow canyons. Black looks at it this way: "If there's a 20 percent chance of rain, do I want to be in there if it's raining hard 20 percent of the time?"