Processes and rates of formation of Holocene alluvial terraces in Harris Wash, Escalante River basin, south-central Utah

Abstract

Harris Wash, a tributary of the Escalante River in south-central Utah, has two well-preserved alluvial terraces of compound origin in its lower reaches. The alluvial stratigraphy of the deposits that compose these terraces reflects the complex processes of canyon filling and erosion. The compound terraces were formed by general aggradation of the valley, by deposition from large floods that also resulted in alluvial-fill incision, and by aggradation of flood-plain surfaces during periods of base-level stability. The alluvial chronology indicates that Harris Wash was aggrading for most of the Holocene and that only two late Holocene periods of rapid incision, between 2500 and 1900 yr B.P. and between 1000 and 300 yr B.P., are evident from the terrace stratigraphy. Rapid aggradation of flood plains during the past 150 yr produced historic flood-plain deposits 5 m above the elevation of the stream. The historic flood-plain deposits and other similar deposits in other Escalante River tributaries are attributed to increased runoff and erosion related to land-use changes in the basin during this time interval. Older alluvial deposits in Harris Wash cannot be correlated with alluvial sequences in other western tributaries of the Escalante River basin. In these drainages, valley aggradation and incision are apparently more sensitive to intrabasin processes of sediment production and storage and less dependent on regional factors such as climate change.