[W]hat makes the Killer Kern so dangerous? It’s a combination of several factors, according to WX Research, a weather and climate research group. One of the river’s defining features is its swift currents, which often reach over 8,000 cubic feet per second. Spring and summer are the most dangerous times, as snowmelt from the Sierra Nevada Mountains adds speed and lowers the water temperature — sometimes to 38 F. Swimmers are at risk of hypothermia or drowning by inhaling water in an instinctive gasp response to the cold. They can also get trapped by underwater hazards or caught in hydraulics, holes that form when the current turns back on itself as it meets an obstacle.