In 2003, the majority of fatal accidents on the lake were related to cliff jumping and diving. Due to the number of fatalities and critical injuries sustained from cliff jumping and diving, it is prohibited for any person to dive or jump off of rock cliffs, ledges, or man-made structures (excluding vessels) within the boundaries of Glen Canyon NRA, including Lake Powell, and its tributaries. (Cliff or ledge is defined as any formation of rock or soil, or combination thereof, having a height of 15 feet or more above the water surface.) The NPS does not advocate or promote the activity of cliff diving/jumping regardless of the height of the cliff feature to the water surface.
As height increases, the risk for serious injury goes up substantially. At ten feet, your body is already traveling about 17 mph when you hit the water. Double that to twenty feet, and you’re going about 25 mph. Even if the water is deep and you go in feet first, your speed is great enough to cause spinal compression, bone fractures, or concussions. Landing even slightly off center can be deadly. The National Park Service prohibits cliff jumping or diving.
Below is a chart representing the speed at which your body will hit the water at given dive distances: