Abstract

The Roaring River alluvial fan formed on 15 July 1982, in Rocky Mountain National Park, Colorado, by a catastrophic flood that was generated by a dam failure. The fan covers an area of 0.25 square km, has a radial length of 0.7 km, and is up to 14 m thick. Sedimentation occurred in three phases, each producing a distinct fan lobe. Initial sedimentation was by a noncohesive sediment-gravity flow which deposited two levees on the proximal boundaries of Lobe I. The levees consist of a poorly sorted mixture of logs, sand, pebbles, cobbles, and boulders. The first two lobes were built primarily by sheetflooding, which deposited imbricated boulders in trains behind obstacles that formed as jams between boulders or logs and upright trees. Horizontally laminated granule and sand sedimentation took place down-fan from the boulders due to deceleration of the expanding sheetflood. Thin-to-medium interbedded sand and cobble-pebble gravel couplets were deposited by sheetflooding on the third lobe. Gravel was transported as bedload by supercritical flow and deposited locally where antidunes broke. Sand was transported as suspended load and deposited where flood velocity locally decreased due to destruction of antidunes, increased roughness, or flow separation around the low-amplitude gravel bed forms. The flood rechannelized at the distal end of Lobe III due to constriction between the fan and the valley margin. Deposits in the upper rechannelized reaches consist of crudely bedded cobble and pebble gravel, and interstratified pebble gravel and backset-bedded sand. Deposition was by supercritical flow. In the lower reaches, planar-cross-bedded, sandy pebble gravel and climbing ripple, horizontal, trough-cross-bedded, and backset-bedded sand were deposited by supercritical and subcritical flow. The flood deposit was modified during waning flood stage and during the three years following the flood by noncatastrophic discharge events. These events formed braided distributary channels by erosion into the top of the sheet flood deposits. Fan building took place mostly by catastrophic unconfined discharge, whereas much of the present fan surface consists of braided channels that formed by erosion into the sheetflood deposits by noncatastrophic discharge.

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