Abstract

Late Holocene terraces in the Coyote Gulch basin, south-central Utah, were formed by aggradation caused by the rapid influx of alluvium produced from landslides on the Straight Cliffs. The main channel of Coyote Gulch was the primary conduit for this sediment and in places aggraded 20 m, burying the preexisting bedrock canyon. This aggradation began about 2300 B.P. and lasted for about 1300 yr. Aggradation in the main stem created a higher base level and reduced the valley gradient in a major tributary, Dry Fork Coyote Gulch, which then partially filled with fine-grained sediment. The major period of aggradation in the Dry Fork was not continuous but was interrupted by incision, presumably when a higher gradient sand-transporting stream was established across this fill. Within the study area more than 25 × 106 m3 of sediment was stored during this aggradational period. Beginning about 900 B.P. this alluvium was incised, creating two terraces in upper Coyote Gulch and as many as three terraces in Dry Fork. Increased sediment production from the erosion of the Dry Fork alluvium caused further deposition downstream in Coyote Gulch, which was eventually incised to form a single terrace. Therefore, rapid sediment loading produced different fluvial responses dependent on the location of a channel segment in the drainage network. Stratigraphic studies can provide insight into the magnitude and rate of sediment storage in stream valleys as well as the timing and processes by which sediment is ultimately transported out of a basin.

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