The magnitude and frequency of tributary debris flows and the historical range of main- stem river discharges are the main factors that create and modify rapids in the Colorado River system. Monitoring of two recently aggraded debris fans in the Green River canyons of the eastern Uinta Mountains shows that main-stem floods with magnitudes between 40% and 75% of the predam 2 yr flood cause significant reworking of fan deposits. Cutbanks formed at fan margins during both small and large flows, indicating that lateral bank erosion is an important reworking mechanism. Armoring of the debris-fan surface limited the degree of reworking by successive floods, even when subsequent flood magnitudes were similar to those that caused significant reworking. Peak discharges increased the width of the reworked zone, decreased fan constrictions, and lowered the water-surface elevation of the ponded backwater. Contrary to predam geomorphic evidence, monitoring indicated that eroded material from recently aggraded debris fans was deposited in bars adjacent to the downstream parts of both fans. This change in the organization of the fan-eddy complex has the potential to alter the location of recirculating eddies and associated areas of fine-grained sediment deposition and storage.

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