Document Type

Thesis

Date of Degree Completion

Fall 2019

Degree Name

Master of Science (MS)

Department

Geological Sciences

Committee Chair

Lisa Ely

Second Committee Member

Breanyn MacInnes

Third Committee Member

Steven Hackenberger

Abstract

The causes and timing of cycles of aggradation and incision in the Hanson Creek drainage in central Washington provide insight into changes in channel morphology and paleoenvironment within the region over the last 8000 years. Stratigraphically and spatially coincident archaeological evidence reveals information related to human occupation during the latter half of the epoch. Using LiDAR imagery and field surveys, recent processes such as degree of modern channel incision, accumulation of valley floor sediment, channel morphology and gradient were evaluated. The spatial distribution of these channel characteristics was assessed in relation to proximal landforms such as colluvial deposits, basalt outcrops, and bedrock anticlinal ridges. Sixteen stratigraphic profiles in the arroyo walls were used to delineate and correlate past depositional episodes based on sediment characteristics. Basal ages of the earliest documented depositional period were constrained using geochemical analysis of tephra beds. Intermediate dates were obtained from 14C analysis of in situ charcoal. The results reveal an aggrading fluvial system that entrained and transported silt to cobble sized sediment from a minimum age of 7680-7580 BP prior to the historic arroyo incision event, which most likely occurred between AD 1878 and AD 1954. The cause of the incision is likely a combination of factors, including thick accumulations of easily erodible sediment in reaches of shallow valley gradient and intervening steep gradients. Although the event that triggered the incision is unknown, similar arroyos in the western U.S. have been initiated by high magnitude floods that incised initial knickpoints, subsequently lowering base level.

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