The Great Cataract - Effects of Late Holocene Debris Flows on Lava Falls Rapid, Grand Canyon National Park and Hualapai Indian Reservation, Arizona

Webb, Robert; Melis, Theodore; Wise, Thomas; Elliott, John

Lava Falls Rapid is the most formidable reach of whitewater on the Colorado River in Grand Canyon and is one of the most famous rapids in the world. Although the rapid was once thought to be controlled by the remnants of lava dams of Pleistocene age, Lava Falls was created and is maintained by frequent debris ﬂows from Prospect Canyon. We used 232 historical photographs, of which 121 were replicated, and 14C and 3 He dating methods to reconstruct the ages and, in some cases, the magnitudes of late Holocene debris ﬂows. We quantiﬁed the interaction between Prospect Canyon debris ﬂows and the Colorado River using image processing of the historical photographs. The highest and oldest debris-ﬂow deposits on the debris fan yielded a 3 He date of 2.9±0.6 ka (950 BC), which indicates predominately late Holocene aggradation of one of the largest debris fans in Grand Canyon. The deposit, which has a 25-m escarpment caused by river reworking, crossed the Colorado River and raised its base level by 30 m for an indeterminate, although probably short, period. We mapped depositional surfaces of 6 debris ﬂows that occurred after 950 BC. The most recent prehistoric debris ﬂow occurred no more than 500 years ago (AD 1434). From April 1872 to July 1939, no debris ﬂows occurred in Prospect Canyon. Debris ﬂows in 1939, 1954, 1955, 1963, 1966, and 1995 constricted the Colorado River between 35 and 80 percent and completely changed the pattern of ﬂow through the rapid. The debris ﬂows had discharges estimated between about 290 and 1,000 m3 /s and transported boulders as heavy as 30 Mg. The recurrence interval of these debris ﬂows, calculated from the volume of the aggraded debris fan, ranged from 35 to 200 yrs. The 1939 debris ﬂow in Prospect Canyon appears to have been the largest debris ﬂow in Grand Canyon during the last 125 years. Debris ﬂows in Prospect Canyon are initiated by streamﬂow pouring over a 325-m waterfall onto unconsolidated colluvium, a process called the ﬁrehose effect. Floods in Prospect Valley above the waterfall are generated during regional winter storms, localized summer thunderstorms, and occasional tropical cyclones. Winter precipitation has increased in the Grand Canyon region since the early 1960s, and the most recent debris ﬂows have occurred during winter storms. Summer rainfall has declined in the same period, decreasing the potential for debris ﬂows in the summer months. The history of river reworking of the Prospect Canyon debris fan illustrates the interrelation between tributary debris fans and mainstem ﬂoods in bedrock canyons. Lava Falls Rapid did not change despite Colorado River ﬂoods of 8,500 m 3 /s in 1884 and 6,230 m3 /s in 1921. Floods up to 3,540 m3 /s that occurred after the historical, pre-dam debris ﬂows removed most of the deposits within 3 years. Releases in 1965 from Glen Canyon Dam that were above powerplant capacity but less than 1,640 m3 /s removed most of the debris fan deposited in 1963, and the combination of...