|Abstract or Summary
- Climate change, combined with population growth, is expected to exacerbate water scarcity globally. In the Columbia River basin (CRB), streamflow is managed for multiple objectives with a network of dams and reservoirs distributed throughout the basin that may mitigate climate change effects on water scarcity. This study quantified trends in daily streamflow from 1950-2012 in 28 pairs of gages above and below dams in the CRB, including the U.S. and Canada. Each gage pair consisted of an above-dam gage that is primarily forested and has little apparent human disturbance and minimal flow regulation or diversions, combined with a below-dam gage immediately downstream of a major dam and reservoir. Monthly streamflow and precipitation for a total of 59 sites was correlated with monthly indices of the Pacific Decadal Oscillation (PDO), the Northern Pacific Index (NPI), the Pacific North America pattern (PNA), and the 700-millibar wind speeds (u700) using Pearson's r. Long-term trends and patterns in daily streamflow and climate data were estimated using linear regression, the Mann-Kendall test, and wavelet analysis. Streamflow was generally weakly correlated with NPI, PNA, and PDO, while positively correlated with upper elevation wind speed at low elevations during wet months. Fewer than half of above-dam gages in the CRB have experienced increasing trends in daily streamflow from mid-March to the beginning of May over the period 1950 to 2012, whereas a majority has experienced decreasing trends from the middle of May to the middle of October. These trends in above-dam basins are consistent with trends expected from climate change, but they also may be affected by legacies of past forest harvest or wildfire. Below dams, reservoir management appears to have overwritten the signal of earlier snowmelt, except in the Boise sub-basin, but long-term declines in late summer flows were evident at half of below-dam sites. Declining summer flows below dams were attributable to a variety of factors, including changes in reservoir management as well as reservoir management that propagates signals from above dam catchments, such as climate change or forest-harvest legacy effects on streamflow. There have been very few significant changes in annual flow volume throughout the basin, and these streamflow trends represent a small percentage of annual flow volume; thus, observed trends appear to be shifts in timing rather than shifts in the annual water balance. The magnitude of 60-yr declines in late summer flow in above-dam catchments represents only a few percent of annual yield, and much less than the storage capacity of the dams, but these long-term changes may be quite important for water yield during the late summer low-flow period. These findings are relevant for strategies to adaptively manage water resources in light of the ongoing planning for renegotiation of the U.S.-Canada Columbia River Treaty.