The Little Colorado River in Arizona, United States, has undergone substantial geomorphic change since the late 1800s and early 1900s, consisting of sediment accumulation following an earlier period of likely widespread sediment evacuation. We analyzed hydrologic and geomorphic data at different spatial and temporal scales to determine the primary mechanisms responsible for these changes, and to provide context for periods of sediment evacuation and accumulation in other rivers.

Peak-flow magnitude has progressively declined since the 1920s despite the occurrence of four alternating periods of high and low total annual flow. Largely coincident with this hydrologic change, the channel has narrowed between 72% and 88% in some reaches since the 1930s, with increases in sinuosity in wide alluvial valleys causing ∼21%–32% reductions in channel slope. Dense stands of vegetation colonized, and thus stabilized, the newly deposited floodplains. Although large, long-duration floods caused some channel widening, these floods have been too infrequent to offset the progressive narrowing. Channel narrowing, increases in sinuosity, decreases in slope, and increases in vegetative roughness appear to have caused biogeomorphic feedbacks, thereby exacerbating sediment deposition, disrupting flood conveyance, and contributing to decreases in peak-flow magnitude and in sediment transport. The progressive increase in water development in parts of the basin has also likely contributed to progressive declines in peak flow. These results show that biogeomorphic feedback processes combined with human water development may be as important as, if not more important than, changes in climate in driving hydrologic, geomorphic, and sediment-load change in dryland river environments.

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