Graduate Thesis Or Dissertation
 

Stream channel stability and sensitivity to landscape history and land use changes in Kelley Creek, Portland, Oregon

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  • This study examines stream channel erosion processes in a small urbanizing watershed influenced by deposits of the Columbia Basin catastrophic floods: Kelley Creek, a 12-km² tributary of Johnson Creek, located just east of Portland, Oregon. Information on landscape history, stream channels, and sediment dynamics was compiled. The effects of future land use changes on stream channel response were projected. The likely effects of engineered solutions (stormwater management detention or infiltration facilities) are compared to land-use controls in mitigating the impact of future development on channel erosion. The geology of Kelley Creek is shaped by volcanic processes and Missoula floods, leading to fine-textured soils and streambeds that are susceptible to erosion. Headwaters are buttes of the Boring Lava formation overlain by Springwater Formation mudflow deposits. Alluvial silts deposited during late Pleistocene catastrophic floods mantle the low-relief valley floor. Land use in Kelley Creek transitioned from forestry to agriculture in the 20th century, and more recently to urban expansion, increasing concerns of watershed managers about effects of development on sediment dynamics. Since 1930, land use on the valley floor of Kelley Creek has been dominated by nursery and berry agricultural production. Forested headwaters have been impacted by roads, culverts, and timber harvest. Seven percent of watershed area is currently impervious, with housing and commercial uses projected to increase following the incorporation of Kelley Creek into Portland's metropolitan urban growth boundary. Field surveys and engineering calculations were used to estimate sediment transport under current and future conditions in the watershed. Cross sectional geometry, slope and sediment size distributions were obtained from 15 locations in first to second-order channels in the watershed, and data were analyzed with sediment transport equations to simulate annual sediment transport rates given current and post-development flow patterns. Results indicate that the stream channel is most sensitive to altered discharge patterns where gravel-sized (D₅₀ > 10 mm) sediments currently are mobilized during annual peak events. In locations where fine (D₅₀ < 5 mm) particles dominate the stream bed, particles mobilize throughout the year and changes in discharge patterns do not increase the duration of transport or overall bedload yields. Because most of the sensitive locations with gravel sediments are located within or near the forested headwaters, preserving these headwater forests will protect sensitive stream channels as effectively as engineered stormwater retention or infiltration approaches. Kelley Creek's stream channel is still adjusting to recent mudflows and catastrophic floods, which provided abundant supplies of fine, erodible materials. Allowing this adjustment to continue without accelerating erosion in the face of rapid urban development poses an unusual challenge for managing geomorphic processes.
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