Effects of inter-annual and decadal-scale climate variability on Winter and Spring streamflow in western Oregon and Washington

River basins in western Oregon and Washington that have a seasonal snowpack exhibit a dual-peaked hydro graph. The early peak is due to the winter maximum rainfall occurring at lower elevations and the later one is due to snowmelt. Three such basins were examined to determine whether the distribution of streamflow within the year is related to indices of either inter-annual (as measured by the Southern Oscillation Index, SOl) or inter-decadal (as measured by the Pacific Decadal Oscillation, PDO) variability in the climate. The relationship of streatnflow to SOl is affected by the phase of the PDO. During below average phases of the PDO (generally warm SST's in the north Pacific Ocean, except along the North American coast). streamflow exhibits a significant relationship to SOl. where as during the above average phase. there is no statistically significant relationship between streamflow and SOl. Statistically significant differences in the distribution of flow within the year were also noted and were related to the phase of the PDO.