Oregon State University

College of Earth, Ocean, and Atmospheric Sciences

Report: Willamette Valley water future mostly bright, though gaps may need to be addressed

Morrison Bridge

12/4/2015

By Mark Floyd

Sources: Anne Nolin; Sam Chan

CORVALLIS, Ore. – During the next 85 years, temperatures in Oregon's Willamette River basin are expected to rise significantly, mountain snowpack levels will shrink dramatically, and the population of the region and urban water use may double — but there should be enough water to meet human needs, a new report concludes.

Fish may not be so lucky. Although ample water may be available throughout most of the year, the Willamette Valley and its tributaries likely will become sufficiently warm as to threaten cold-water fish species, including salmon and steelhead, the scientists say.

These are among the key findings of the Willamette Water 2100 Project, a five-year, $4.3 million study funded by the National Science Foundation and led by Oregon State University, in partnership with researchers from the University of Oregon, Portland State University and University of California at Santa Barbara.

"The Willamette River basin today is characterized by abundant annual water and sometime seasonal shortages," said Anne Nolin, an OSU professor of environmental sciences and principal investigator on the study. "That should continue into 2100, despite much warmer temperatures, more people and a substantial loss of snowpack.

"The reason for optimism is the region's 11 storage reservoirs coordinated by the Army Corps of Engineers that act as a valve for seasonal differences and preserve water for times of need," Nolin added. "Without them, the picture would look quite a bit different."

Analysis of global circulation models suggest that the Willamette River basin will warm between two and 13 degrees (Fahrenheit) by the year 2100, thus scientists used three separate scenarios to look at potential impacts based on low, medium and high rates of temperature increase. These temperature increases will result in a dramatic decline in snowpack — from 63 to 95 percent lower than average — changing seasonal water flow patterns.

Scientists also explored results from a range of population, economic and policy scenarios that allowed them to ask "what if?" questions for different human changes and interactions with climate changes. Much of the climate modeling for the project was developed through a regional integrated sciences and assessments (RISA) program at Oregon State, which is funded by NOAA and led by OSU Professor Philip Mote.

There is little doubt that temperatures will increase, the report notes, but there is less certainty about the impact of a changing climate on precipitation. Winters may actually be slightly wetter, though more of the precipitation will fall as rain instead of snow. Summers should be drier, necessitating more reliance on water held behind the region's 11 storage reservoirs.

"Although there are a number of government entities — federal and state — involved in regulating water use from those reservoirs, there appears to be enough flexibility in the system to adequately adapt for changing conditions in the future," said Nolin, a professor in OSU's College of Earth, Ocean, and Atmospheric Sciences.

The report notes that warmer temperatures, less snowpack and drier summers will greatly increase the danger of wildfire in the mountains feeding the Willamette River basin — by about 200 to 900 percent. Their simulations show that fire will open up lands to new forest types and reduce the availability of forestland for timber harvest.

Increasing urban use of water from a population that could double will involve costly expansions in infrastructure. As the population grows, more agricultural land near urban areas will be developed for housing and other needs, according to Samuel Chan, a watershed health specialist with Oregon Sea Grant and the broader impacts outreach lead for the Willamette Water 2100 Project.

However, the report shows that in some cases where urban areas are expanding into what are now irrigated farmlands, these locations may see a net decline in water use.

"The report notes the difference between water 'diversions' and water 'consumptive use,'" Chan noted. "As the population grows, the need for water will increase, but much of it will be used, and then treated in wastewater plants and returned to the system. Other uses, like forests and agriculture, consume the water through evaporation and transpiration to the atmosphere."

"The downside, though, is that treated water that is returned to the river is often warmer, increasing the impact on cold-water fish species," he added.

The main drivers for changing water needs, the report concludes, are climate change, and growth in population and income.

"The dams built above the Willamette Valley were engineered for reducing the risk of floods, but they also do a valuable job in storing water for use during summer," Nolin said. "They can store large amounts of water in the summer, when they are not kept empty for flood prevention and there is existing flexibility in water allocation policies that could help western Oregon adapt to a climate that may be quite different in the future."

"Unlike many parts of the country, those of us who live in the Willamette Valley are lucky because we have abundant water for human use, and we have institutional capacity to help mitigate water scarcity," she added. "However, the biggest negative impacts are likely to be for native cold-water fish and we will likely be facing a significant challenge in managing stream temperature for fish."


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