Climate-Driven Changes in Synchronicity of Mainstem and Tributary High Flows May Alter Flood Risk in the Columbia River Basin

Abstract:

Projections of change in high-flow extremes with global warming vary widely among mid-latitude rivers of the world. In general, headwater rivers often show relative increases in magnitude that are much greater than those of the much larger rivers they feed. Several reasons have been proposed for this difference between large and small rivers, but one that has received very little attention is the role of changing synchronicity in high-flow extremes on large rivers and their tributaries. Changes in synchronicity can either dampen or amplify downstream changes in high flows. Synchronicity can change as flow regimes among tributaries respond to climate change at varying rates within a large basin that is climatically heterogeneous. We examine mainstem-tributary synchronicity during high-flow extremes in naturalized historical records and a large ensemble of simulated streamflow in the Columbia River Basin in the Pacific Northwestern United States and Canada. We estimate the extent that changing synchronicity explains the relatively small changes in projected high-flow extremes on the downstream reaches of the Columbia and Snake Rivers, despite large increases at headwaters.