STREAM CHANNEL STABILITY AND SENSITIVITY TO LANDSCAPE HISTORY AND LAND USE CHANGES IN KELLEY CREEK, PORTLAND, OREGON

This study examines stream channel erosion processes in an urbanizing watershed influenced by Columbia Basin catastrophic flood deposits: Kelley Creek, a 12-km² tributary of Johnson Creek, located east of Portland, Oregon. We compiled information on landscape history, stream channels and sediment, and projected stream channel response to future land use changes. We compared the likely effects of engineered solutions (detention or infiltration) versus land-use controls in mitigating development’s impacts on channel erosion.

Volcanic processes and Missoula floods shaped the geology of Kelley Creek, leading to fine-textured soils and erosion-susceptible streambeds. Headwaters are Boring Lava formation buttes overlain by Springwater Formation mudflow deposits; silts deposited by late Pleistocene catastrophic floods mantle the valley floor.

Settlers began farming Kelley Creek’s valley in the late 19^th century. More recently, urbanization is increasing concerns of development’s effects on sediment dynamics. Forested headwaters are impacted by roads and logging. 7% of watershed area is currently impervious, with housing and commerce increasing following incorporation into Portland’s urban growth boundary.

Field surveys and engineering calculations were used to estimate sediment transport under current and future conditions. Cross sectional geometry, slope and sediment size distributions were obtained from 15 locations in first to second-order channels. We analyzed data with sediment transport equations to simulate annual sediment transport rates given current and post-development flow patterns.

Results indicate that the channel is sensitive to altered flow patterns where gravel-sized (D­₅₀ > 10 mm) sediments currently mobilize during annual peak events. Where silts (D₅₀ < 5 mm) dominate the stream bed, particles mobilize throughout the year and flow pattern changes do not increase transport duration or bedload yields. Because most of the sensitive locations are in forested headwaters, preserving these forests will protect sensitive channels as effectively as engineered approaches.

As Kelley Creek adjusts to mudflows and catastrophic floods, managers are faced with the unusual challenge of allowing adjustment to continue while not accelerating erosion with urban development.