Ubiquitous increases in flood magnitude in the Columbia River basin under climate change

Abstract

Abstract. The USA and Canada have entered negotiations to modernize the Columbia River Treaty, signed in 1961. Key priorities are balancing flood risk and hydropower production, and improving aquatic ecosystem function while incorporating projected effects of climate change. In support of the US effort, Chegwidden et al. (2017) developed a large-ensemble dataset of past and future daily streamflows at 396 sites throughout the Columbia River basin (CRB) and selected other watersheds in western Washington and Oregon, using state-of-the art climate and hydrologic models. In this study, we use that dataset to present new analyses of the effects of future climate change on flooding using water year maximum daily streamflows. For each simulation, flood statistics are estimated from generalized extreme value distributions fit to simulated water year maximum daily streamflows for 50-year windows of the past (1950–1999) and future (2050–2099) periods. Our results contrast with previous findings: we find that the vast majority of locations in the CRB are estimated to experience an increase in future streamflow magnitudes. The near ubiquity of increases is all the more remarkable in that our approach explores a larger set of methodological variation than previous studies; however, like previous studies, our modeling system was not calibrated to minimize error in maximum daily streamflow and may be affected by unquantifiable errors. We show that on the Columbia and Willamette rivers increases in streamflow magnitudes are smallest downstream and grow larger moving upstream. For the Snake River, however, the pattern is reversed, with increases in streamflow magnitudes growing larger moving downstream to the confluence with the Salmon River tributary and then abruptly dropping. We decompose the variation in results attributable to variability in climate and hydrologic factors across the ensemble, finding that climate contributes more variation in larger basins, while hydrology contributes more in smaller basins. Equally important for practical applications like flood control rule curves, the seasonal timing of flooding shifts dramatically on some rivers (e.g., on the Snake, 20th-century floods occur exclusively in late spring, but by the end of the 21st century some floods occur as early as December) and not at all on others (e.g., the Willamette River).

Greater Yellowstone climate assessment: past, present, and future climate change in greater Yellowstone watersheds

“…occur; however, as the susceptible range of snow rises to higher elevations under warming, the number of events could increase by an additional day or two in the future (Mussleman et al 2018). Queen et al (2021) found that warming and a shift to more rain-dominated precipitation will likely extend the flood season on the Upper Snake River around Jackson WY (presently from mid-May to mid June), to earlier in the year and increase the magnitude of large floods (10-year and 100-year recurrence interval).…”

Changing River Network Synchrony Modulates Projected Increases in High Flows

“…Although Queen et al. (2021) do not show detailed results from RCP4.5, they state that that “to first order the changes in flood magnitude in RCP4.5 were ∼2/3 those in RCP8.5.” The Columbia River, along much of its mainstem, is an important exception to their generalization and the cause for the nonmonotonic response to increased anthropogenic forcing may lie in changes in mainstem‐tributary synchrony, which we discuss in Section 3.4.…”

“…AMFs with relatively low synchrony also occur more frequently in the future scenarios, extending the lower tail of the distribution of α (e.g., Figures 3b and 3c). One explanation for the decreased synchrony is the region‐wide shift toward an earlier freshet occurring at different rates among tributaries (Queen et al., 2021). Because the snowpack in the colder headwaters, such as the upper Columbia headwaters in the Canadian Rockies, is less impacted by the projected warming than the snowpack in warmer headwaters that tend to feed the downstream tributaries (e.g., Chegwidden et al., 2019), the relative timing of the freshet can be expected to diverge across many tributaries to the Columbia.…”

“…Queen et al. (2021) speculated that the relatively large projected increases in headwater basins do not persist to the downstream reaches of the Columbia because downstream reaches integrate a larger diversity of hydroclimates and individual weather events. However, Queen et al.…”

“…However, Queen et al. (2021) did not describe specifically how greater hydroclimatic diversity leads to a decreased sensitivity in high‐flow response to anthropogenic warming, leaving the matter unresolved.…”