A recent debris flow in Monument Creek illustrates the nature of debris flows in small tributaries and their hydrologic effects on the Colorado River in Grand Canyon National Park. A debris avalanche originated in the Permian Esplanade Sandstone of the Supai Group during intense rainfall on July 27, 1984, and fell 600 m into Monument Creek, forming a 7-m-high barrier across the channel. The subsequent debris flow traveled 4.5 km to the Colorado River and achieved velocities of 3.4 to 4.0 m/s and a peak discharge of 100 to 120 m3/s. The flow consisted of a main pulse followed by subsequent pulses of debris flow or hyperconcentrated flow. The main pulse moved boulders as large as 2.7 m in diameter, and deposition at the mouth of Monument Creek enlarged the fan surface and significantly constricted the Colorado River. Most of the major rapids on the Colorado River in Grand Canyon National Park appear to be maintained by episodic debris flows.