But below the confluence with the Salmon River on the Oregon-Idaho border, the increase drops to 20% for 10-year floods and 30% for 100-year floods. Parts of the Snake River will see a 40% increase in 10-year floods and a 60% increase in 100-year floods. On the Snake River, streamflows will grow larger as they move downstream until they reach the confluence of the Salmon River tributary and then will drop abruptly. The streamflows are expected to be smaller downstream and grow larger upstream. The Willamette River and its tributaries are expected to see the biggest increase in flooding magnitude, with 50% to 60% increases in 100-year floods. "It shows that the magnitude of one-, 10- and 100-year floods is likely go up nearly everywhere in the region. "This was the best and most complete set of data," he said. Previous studies predicting future streamflows showed mixed results, but the results of this new analysis were clear and surprising, Mote said. The data included a 50-year window from the past, 1950-1999, as well as a 50-year window of expected streamflows in the future, 2050 to 2099, that was developed using several different climate models. Queen ran simulations using hydrology models and a previously collected set of streamflow data for 396 sites throughout the Columbia River basin and other watersheds in western Washington. Floods on the Chehalis River in 20 closed Interstate 5 in Washington and floods along the Willamette River in 19 caused hundreds of millions of dollars in damage. The largest flood in modern history occurred in late spring 1948 when flooding from the Columbia River destroyed the city of Vanport, Oregon, displacing more than 18,500 people. The Pacific Northwest has a history of costly and disruptive flooding. It has the fourth-largest streamflow volume in the United States. The Columbia River drains much of the Pacific Northwest, including portions of seven states and British Columbia. The goal of Queen's research was to better understand how flooding in the Columbia River basin might change as the planet warms. Queen, a Corvallis native, continued the work at OCCRI and is now enrolled in a doctoral program at Victoria University of Wellington in New Zealand. The study emerged out of Queen's work on her honors thesis as an undergraduate in the University of Oregon's Robert D. Co-authors are David Rupp of the Oregon Climate Change Research Institute and Oriana Chegwidden and Bart Nijssen of the University of Washington. The findings were published recently in the journal Hydrology and Earth System Science. "But managing a 30% to 40% increase, as is predicted for many areas, is clearly beyond our management capabilities." "We don't know how much of this increased flood risk can be managed through mitigation measures until we study the issue further," Mote said. The findings are based on natural river conditions and do not take into account potential flood control measures, including dams, but the increases are significant nonetheless, said study co-author Philip Mote, a professor in the College of Earth, Ocean, and Atmospheric Sciences and dean of the Graduate School at OSU. "The flood you're used to seeing out your window once every 10 years will likely be larger than it has been in the past," said the study's lead author, Laura Queen, a research assistant at OSU's Oregon Climate Change Research Institute. In some areas, the flooding season will expand, as well. The magnitude of flooding - the term used to describe flooding severity - is expected to increase throughout the basin, which includes the Columbia, Willamette and Snake rivers and hundreds of tributaries.
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