Distinguishing human and climate influences on the Columbia River: Changes in mean flow and sediment transport

► Human and climate influences on the Columbia River hydrologic cycle and sediment discharge have been separated since 1858. ► The mean virgin flow at The Dalles has decreased ∼16.5%, 8-9% due to climate change and 7-8% due to water withdrawal. ► Climate impacts on the sediment discharge are larger than on streamflow because it increases more than linearly with flow. ► Flow regulation has decreased peak spring flows by ∼45%; freshet peak occurs now 2-4 weeks earlier than before 1900. ► Spring freshet flow decrease due to climate change, water withdrawal and flow regulation are 11%, 12% and 26% respectively. Most hydrologic trends result from a combination of climatic and human influences, and analyses of hydrologic changes often do not differentiate these factors, despite the obvious relevance of this distinction. Here, we separate human and climate influences on the Columbia River hydrologic cycle and sediment discharge on the basis of robust data analyses since 1858. Human influences include water withdrawal for irrigation, flow regulation, reservoir manipulation, mining and deforestation. The Columbia’s streamflow and sediment discharge are strongly correlated with large-scale climate patterns, particularly the ENSO (El Niño Southern Oscillation) and PDO (Pacific Decadal Oscillation). The mean annual Columbia River virgin flow at The Dalles has decreased ∼16.5%, 8–9% due to climate change and 7–8% due to water withdrawal for irrigation. Climate impacts on the sediment discharge are larger than on streamflow because sediment discharge increases more than linearly with flow. Total sediment and sand transports have decreased >50% and >70% respectively, only a fraction of which is due to climate change. Changes in the timing of maximum flows from sub-basins, as influenced by flow regulation and irrigation withdrawal, determine freshet timing and play a larger role in determining the maximum flow and sediment transport levels. Flow regulation (since 1970) has decreased peak spring flows by ∼45% and increased flow during the rest of the year. The spring freshet flow decrease due to climate change is 11%; the decreases due to water withdrawal and flow regulation are about 12% and 26% respectively. The peak freshet flow now typically occurs 2–4 weeks earlier than before 1900.