The 1983 Hydraulic Jump in Crystal Rapid: Implications for River-Running and Geomorphic Evolution in the Grand Canyon

For the last 1, 000 to 10, 000 years, dozens of large debris fans have severely constricted the path of the Colorado River in the Grand Canyon, Arizona. At most of these fans, the narrowest part of the channel eroded by the river is 0. 5 of the upstream width. At Crystal Creek, a debris fan was emplaced in 1966, constricting the channel of the Colorado River to about 0. 25 of its upstream width between 1967 and 1983, forming a major rapid. In this paper the hydraulics of Crystal Creek rapid are described, and an analysis is presented to support the hypothesis that the major wave in the rapid was a normal wave (one type of hydraulic jump). Hydraulic jumps rarely occur in natural river channels with erodible beds, but one was present at Crystal Rapid because of the unusually severe constriction of the Colorado River by the 1966 debris fan. Observations on the hydraulics of the river during this time (including mid-1983, when progressively higher discharges culminated in excess of 96, 000 cubic feet per second) have demonstrated that the velocity of water going through the constriction and into the hydraulic jump was so great that there was erosion of the Crystal debris fan in the vicinity of the jump. Each new level of record high discharges caused the river to erode a channel of sufficient width to reduce flow velocities below a threshold value required for movement of the larger boulders of the debris fan, thus contouring the fan toward a configuration more in equilibrium with the high discharges. A quantitative model for river debris fan shapes is proposed and is used to estimate prehistoric flood levels from the observed constrictions: the 0. 5 value of river constriction found at the more mature debris fans in the Grand Canyon suggests that peak flood discharges of approximately 400, 000 cubic feet per second (11, 320 m^3/s) have occurred.