Climate Change in the Pacific Region
Pacific Region

Stream Flow Changes in Pacific Northwest Freshwater Systems

Timing and amount of streamflow is changing. The decrease in spring snowpack and earlier snowmelt has led to a change in streamflow in many systems, including earlier spring runoff peaks, increased winter streamflow, and reduced summer and fall streamflows when water temperatures are at their highest (Stewart et al. 2005; Arismendi et al. 2013; Safeeq et al. 2013). Rain-dominated systems and systems with higher groundwater base flow are less sensitive. Stewart et al. (2005) for example, examined 302 streamflow gages in the western United States and reported that the timing of winter runoff and annual streamflow had advanced by one to four weeks from 1948 to 2002. The degree of change depends on the location and elevation of the specific river basin. Basins located significantly above freezing levels have been much less affected by warmer temperatures than those located at lower elevations. River basins whose average winter temperatures are close to freezing are the most sensitive to climate change, as is apparent from the dramatic shifts in streamflow timing that have resulted from relatively small increases in wintertime temperatures. The advance in streamflow timing also results in decreased summer and fall base flows, at precisely the time when streamflow is needed most. In addition, warmer temperatures have lengthened the growing season (defined as the time between the last frost of spring and the first frost of fall) in the western United States by an average of about 10 to 15 days. Warmer temperatures and longer growing seasons also increase water requirements for evapotranspiration, hydropower, irrigation and other uses, resulting in potential water supply shortages and conflicts.



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Last updated: September 12, 2013

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