PROCESSES AND TIMING OF HOLOCENE DEPOSITION, CHANNEL INCISION AND COINCIDENT ARCHAEOLOGY, HANSON CREEK, WASHINGTON

The causes and timing of cycles of sediment deposition and erosion in the Hanson Creek drainage in central Washington provide insight into changes in channel morphology and paleoenvironment within the region since the mid-Holocene (8 kya-present). 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 land forms such as spring mounds, 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.

Stratigraphic evidence suggests a transition from an aggrading braided system to an expansive, fine grained, alluvial step-pool sequence that continually aggraded throughout the mid- to late- Holocene. A single, 9-m deep incision of a 1.8 km reach occurred around AD 1900. Low gradient, fine grained, organic-rich, sediment suggests prolonged periods of ponding at three locations throughout the incised reach, adjacent to evidence of ground-water springs. The intervening reaches exhibit comparatively high gradients for unconsolidated alluvium.

The chronological and paleoenvironmental data was correlated with the abundant lithic and anthrosol evidence to provide a comprehensive understanding of the relationship between the fluvial environment and the timing, distribution, and preservation of human occupation sites near Hanson Creek throughout the Holocene.The timing and physical environmental conditions associated with the deposition and erosion of Holocene sediment in the Hanson Creek watershed will supplement the minimal data available on arroyo formation in the northwestern U.S. and allow a comparison with previous alluvial chronologies from the region.