Climatic and physiographic controls on peakflow generation in the western Cascades, Oregon

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Perkins, Reed M. 1997. Climatic and physiographic controls on peakflow generation in the western Cascades, Oregon. Corvallis, OR: Oregon State University. 190 p. Ph.D. dissertation.


Despite the importance of rain and snowmelt in causing peak flows in the Pacific Northwest, the interaction effects of a snowpack and watershed physiography on streamflow are largely undocumented. This study investigated the influence of soil and snowpack moisture on peakflow hydrograph shape in three small (<60 ha) control sub-watersheds and the containing Lookout Creek Watershed (6200) ha in the western Cascade mountains of Oregon with three main objectives: (1) determine the statistical correlation between antecedent conditions and peakflow hydrograph shape within each small watershed, (2), determine how these correlations differed between small watersheds, and (3) determine the correlation between sub-watershed and Lookout Creek peakflow hydrograph shapes. A partial-duration set of peakflows was selected from at least 21 years of continuous discharge data for each small watershed using a linear quickflow vs. baseflow separation technique. Peakflows were then classified into three significantly different types based on estimated antecedent soil and snowpack moisture storage: "dry-soil rain" (e.g. soil moisture < 90% field capacity), "wet-soil rain (e.g. soil moisture _90% field capacity), and "wet-soil rain-on-snow," These three types accounted for over 70% of peakflows and over 80% of quickflow volume in each watershed. Generally, wet-soil rain peakflows began sooner after precipitation, lasted longer, and had larger instantaneous peaks than did dry-soil rain peakflows, while wet-soil rain-on-snow peakflows had the longest durations and the largest instantaneous peaks. However, effects of soil wetting and snowpack on peakflow hydrographs differed significantly between watersheds. Lookout Creek peakflows occurred during several combinations of small watershed peakflow types, though the largest Lookout Creek peakflows were coincident with large, synchronized wet-soil rain-on-snow peakflows in Watersheds 2 and 8. Very little attenuation of the peakflow hydrograph was observed between the small watersheds and Lookout Creek. Results from this study indicate that by altering snowpack dynamics, and therefore the frequency distribution of peakflow types, climate change or land-use may differentially affect peakflow hydrographs in small watersheds. On consequence of this in Lookout Creek watersheds may be to desynchronize small watershed peakflow responses and decrease downstream peak size.