Bankfull discharge recurrence intervals and regional hydraulic geometry relationships: Patterns in the Pacific Northwest, USA

The model bankfull discharge recurrence interval (annual series) (T-a) in streams has been approximated at a 1.5-year flow event. This study tests the linkage between regional factors (climate, physiography, and ecoregion) and the frequency of bankfull discharge events in the Pacific Northwest (PNW). Patterns of Ta were found to be significant when stratified by EPA Ecoregion. The mean value for T-a in the PNW is 1.4 years; however, when the data is stratified by ecoregion, the humid areas of western Oregon and Washington have a mean value of 1.2 years, while the dryer areas of Idaho and eastern Oregon and Washington have a mean value of 1.4 to 1.5 years. Among the four factors evaluated, vegetation association and average annual precipitation are the primary factors related to channel form and T-a. Based on the results of the Ta analyses, regional hydraulic geometry relationships of streams were developed for the PNW, which relate variables, such as bankfull cross-sectional area, width, depth, and velocity, to bankfull discharge and drainage area. The verification of Ta values, combined with the development of regional hydraulic geometry relationships, provides geographically relevant information that will result in more accurate estimates of hydraulic geometry variables in the PNW.