Climate Resiliency

According to the 2014 Report, since around 1950 the average snowpack on April 1 in the Cascade Mountains decreased about 20%, spring snowmelt occurred 0–30 days earlier depending on location, late winter/early spring streamflow increases ranged from 0–20%, and summer flow decreased 0–15% in the Pacific Northwest. By 2050, snowmelt is projected to shift three to four weeks earlier than the 20th century average, and summer flows are projected to be substantially lower.

Reduced summer flows, combined with increased water temperatures and increased demand from multiple users, will pose significant challenges for maintaining in-stream flow targets that ensure survival of Columbia River salmonids. During winter months, when more precipitation is expected to fall as rain instead of snow, increased flows may lead to increased flooding and erosion concerns. Shifts in flow timing can also adversely impact agriculture, and in particular irrigated crops that rely on groundwater and/or reservoir storage with limited capacity.

While the 2014 Report is uncertain about the overall change in precipitation amounts, it does point to an expected sharp decrease in summertime precipitation, of as much as 30% by the end of the century. Coupled with an already dry summer climate, this could have profound implications for the types of vegetation that will dominate in the future.

Extreme winter precipitation events which are expected to occur more frequently may lead to more severe flooding, risk to infrastructure, and decreased water quality as more sediment and pollutants become mobilized. Increased sediment mobility and flows resulting from these events can in turn bury salmonid eggs and reduce salmonid survival.

A shift in precipitation towards increased rain and decreased snow is expected as average atmospheric temperatures continue to increase. This will result in reduced snowpack in many areas, which will impact the timing and magnitude of river flows as snows melt. It may also exacerbate water temperature increases, and change the quantity and quality of spawning habitat for various salmonid species that utilize snowmelt driven streams.

Summertime water temperatures in the mainstem Columbia River have been rising for decades, and will continue to rise as the world continues to exceed global temperature records. As shown by the US Forest Service's NorWeST Stream Temperature Model, rivers and streams throughout the Northwest are similarly threatened.

Increased water temperatures will impact the estuary ecosystem in many ways. Salmonid species, all of which rely on cold water and become physically and metabolically stressed when exposed to water above certain temperatures, are directly threatened. Even today, mainstem Columbia River temperatures regularly exceed these thresholds for several weeks during the summer. The situation is expected to worsen as temperatures, as well as the duration of time that fish are exposed to lethal temperatures, continue to increase. According to the 2018 report, "projections for increased stream temperature indicate a 22% reduction in salmon habitat in Washington by late century under a high emissions future." Increased water temperature is also expected to reduce other aspects of water quality. Warmer temperatures decrease dissolved oxygen content, which is vital to the survival of aquatic organisms, and promote harmful algal blooms such as cyanobacteria, which have already become more prevalent in recent years. Finally, habitat for non-native, warm-water species will increase, increasing competition with, and predation on, native cold-water species. 

According to the Washington Coastal Resiliency Project's 2018 Sea Level Rise Assessment, sea level is likely to rise from between 0.2 and 2.0 feet by 2100 at the mouth of the Columbia River, depending on what emission's scenario unfolds. Sea level rise presents several risks to coastal communities, through accelerated erosion and increased flooding from tides and storm surge or wind-driven events. All of these can put infrastructure at greater risk. As stated in the 2018 National Climate Assessment report: "As the pace and extent of coastal flooding and erosion accelerate, climate change impacts along our coasts are exacerbating preexisting social inequities, as communities face difficult decisions about determining who will pay for current impacts and future adaptation and mitigation strategies and if, how, or when to relocate."

A potentially important ecological threat from sea level rise in the estuary is the loss of wetlands habitat due to increased floodplain inundation. With nearly 80% of the estuary's historical wetlands already lost to development and other human impacts, knowing how much more will be lost due to sea level rise, and how much of that additional loss may be potentially offset by wetlands formation elsewhere, is a critical piece of information for estuarine ecosystem management.