Cosmogenic He-3 ages and frequency of late Holocene debris flows from Prospect Canyon, Grand Canyon, USA

Lava Falls Rapid, which was created and is maintained by debris flows from Prospect Canyon, 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. D ebris flows enter the Colorado River at tributary junctures, creating rapid s. The frequency of debris flows is an important consideration when managem ent of regulated rivers involves maintenance of channel morphology. We used cosmogenic He-3, C-14, and historical photographs to date 12 late Holocene and historic debris flows from Prospect Canyon. The highest and oldest dep osits from debris flows on the debris fan yielded a He-3 date of about 3 ka , which indicates predominately late Holocene aggradation of one of the lar gest 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 l evel by 30 m for an indeterminate although likely short period. We mapped d epositional surfaces of 11 debris flows that occurred after 3 ka. Two depos its inset against the highest deposit yielded He-3 ages of about 2.2 ka, an d at least two others followed shortly afterwards. At least one of these de bris flows also dammed the Colorado River. The most recent prehistoric debr is flow occurred no more than 0.5 ka. The largest historic debris flow, whi ch constricted the river by 80%, occurred in 1939. Five other debris flows occurred after 1939; these debris flows constricted the Colorado River by 3 5-80%. Assuming the depositional volumes of late Holocene debris flows can be modeled using a lognormal distribution, we calculated recurrence interva ls of 15 to more than 2000 years for debris flows from Prospect Canyon. (C) 1999 Elsevier Science B.V. All rights reserved.