It is axiomatic that water defines a river, but it is particularly so for the Columbia River, where the unique geography and climate of the river basin contribute to the tremendous volume of annual runoff — fourth-largest in North America behind Mississippi, Mackenzie, and St. Lawrence. Historic precipitation patterns influence ecological processes such as the location and abundance of fish and wildlife populations, human uses of the land, and regimes of fire and flood.
The climate west of the Cascade Mountains is influenced by the proximity of the Pacific Ocean. Winters are mild, and rainfall is frequent and at times heavy. Snowfall is rare west of the Cascades except at the higher elevations of those mountains and in the Coast Range. In the low-lying areas of the Columbia and Willamette river valleys snow usually only occurs within a period of weeks in January and February. The climate east of the Cascades is markedly different, a continental climate of cold, snowy winters and warm, dry summers. The continental climate extends into British Columbia, where successive mountain ranges also catch winter storms and snowfall is frequent and heavy, but summers generally are warm and dry.
Annual precipitation varies from 45 inches in the Portland/Vancouver metropolitan area, to 100 inches or more in the heart of the Columbia River Gorge, to fewer than 8 inches in the rain shadow area immediately east of the Cascades. Most of the interior Columbia River Basin receives between 12 and about 30 inches of precipitation annually, with the exception of the Snake River Plain in southern Idaho, which is drier.
The mountain ranges within the Columbia River Basin typically receive 100 to 200 inches of snow annually; this is one reason for the Columbia’s huge annual runoff volume, which averages 192 million acre-feet. The annual spring freshet, when the snow melts and runoff peaks in the spring and early summer, historically provided the transit to the estuary and ocean for juvenile salmon and steelhead. Dams altered this flow regime by catching and holding water in reservoirs for winter power generation, thus reducing the volume and velocity of the spring flow.
Climate also affects growing seasons in the Columbia River Basin. At higher elevations, the season can last as few as 30 days. In the intermontane valleys and plateaus, however, such as the interior plateau that stretches mostly unbroken from central Washington to central Oregon, and in the comparatively damp Willamette Valley of Oregon, growing seasons are 150 to 200 days long.
Air quality in the Columbia Basin is heavily affected by wind, and the interior parts of the basin can be very windy. The average wind speed in the Columbia River Gorge, where the steep walls can act as a funnel, is 10 miles per hour. This is primarily why the Gorge has become so popular with wind surfers. Frequent and sustained winds of 20 miles per hour or faster are common. Farther to the east, in central and southeastern Washington and northern and northeastern Oregon, the fine-grained loessal soils are particularly susceptible to wind erosion, and periodic dust storms in the spring and summer can raise clouds of roiling dust that halt traffic on the inland highways. It is not unusual for eastern Washington to be blanketed with dust from spring and summer winds from the west. The frequency of sustained winds has prompted a renaissance of wind-power developments in southeastern Washington and northeastern Oregon. Long rows of 100 or more spinning turbines are becoming an increasingly common sight on the mostly treeless ridgetops.
Periodically the Gorge funnels wind, hot in summer and cold in winter, into the Portland/Vancouver area, the largest population center in the Columbia River Basin, with dramatic effects. In summer, the hot wind can blow temperatures to over 100 degrees F and cause a marked drop in humidity in the Portland area. The winds continue down the river all the way to Astoria, where the temperature might climb into the 80s or 90s. Yet just a few miles south of Astoria, where the river wind doesn’t reach, it will be 30 degrees cooler and foggy. In winter, cold winds blowing down the Gorge can chill the Portland area to well below freezing. When a warm, wet weather system overrides the surface-level cold air, the temperature difference between the surface and about 2,000 feet in elevation can be 15 or 20 degrees — enough to keep the inevitable rain from turning to snow before it hits the ground. These storms can be vicious, dumping up to 10 feet of snow in the western end of the Gorge, although that has not happened since the early 20th century, but more commonly laying down a coating of solid ice. Such a storm paralyzed the Portland area for four days in January 2004.
Climate change impacts and the Columbia River Basin
A significant proportion of scientific opinion, based on both empirical data and large-scale climate modeling, holds that the Earth is warming due to atmospheric accumulation of carbon dioxide (CO2), methane, nitrous oxide, and other greenhouse gasses. The increasing atmospheric concentration of these gasses appears to be largely from human activities, in particular, the burning of fossil fuels. The effects of warming may include changes in atmospheric temperatures, storm frequency and intensity, ocean temperature and circulation, and the seasonal pattern and amount of precipitation. Possible beneficial aspects to warming, such as improved agricultural productivity in cold climates, on balance appear to be outweighed by adverse effects such as increased frequency of extreme weather events, flooding of low-lying coastal areas, ecosystem stress and displacement, increased frequency and severity of forest fires, and northward migration of warm-climate diseases. While there is general agreement on the fact and impacts of global warming, significant uncertainties remain regarding the rates and ultimate magnitude of warming and its effects.
In its Fifth Northwest Power Plan, the Council addresses potential impacts of climate change. Two appendices, Appendix M and Appendix N, address global climate change policy and effects of global climate change on the Columbia River Basin hydroelectric system, respectively. Global models seem to agree that Northwest temperatures will be higher, but they disagree regarding levels of precipitation. Current thinking by Northwest scientists leans toward a warmer and wetter climate. The proportion of winter precipitation currently falling as high-elevation snow is expected to decline, and peak runoff is expected to shift from springtime to winter. Summer stream flows would decline as a result of loss of snow pack. Warming would lead to a relative reduction in winter peak electricity demand and an increase in the frequency and intensity of summer peaks.