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Show Chapter 2 The River and Its Tributaries The Columbia River rises in Columbia Lake, just west of the Continental Divide in the southeastern part of British Columbia, Dominion of Canada. The basin which it drains extends southward, and is broadened by inclusion of the Snake River Val- ley in southern Idaho and Oregon. The distance across the southern edge is some 700 to 800 miles. The river drains about 180 million acre-feet of water from more than a quarter of a million square miles of land, nearly 85 percent of which lies with- in the United States. The amount of runoff from the Columbia Basin is exceeded in the United States only by that from the Mississippi system. In Can- ada, the river flows 190 miles northwest from its source, and thence generally southward 270 miles to the international boundary. Within the United States the river continues for about 745 miles. At no point in its entire course is the Columbia main stem more than about 200 miles from the western edge of the basin. The Columbia has a fall of about 2,700 feet from its source in Columbia Lake to tidewater. As a result, through most of its length the main stem and its tributaries' lower reaches flow in deeply cut stream beds. Many of the tributaries have even greater fall, a drop of 3,000 to 6,000 feet being common. The Snake River is the principal tributary of the Columbia in terms of length and drainage area. It drains approximately the southeastern half of the American portion of the Columbia Basin. It contributes about 37 million acre-feet or approxi- mately one-fifth of the total runoff of the Columbia. There are three principal hydrologic divisions of the Columbia Basin: (1) The region west of the crest of the Cascade Mountains, (2) the great cen- tral area east of the Cascade Range, comprising the Columbia Plateau in the north, and the Snake River Plains in the south and southeast, and (3) the northern Rockies and mountain valleys in the northeast (figure 1). These three regions make very different contribu- tions to the total runoff of the basin. West of the Cascades, nearly one quarter of the total runoff of the Columbia comes from less than one-tenth of the land area of the basin. In the northern Rocky Mountain section in the United States and the Dominion of Canada, nearly half of the total flow of the Columbia is drawn from slightly more than one-quarter of the land areas of the basin, while in the central region, 60 percent of the land area con- tributes less than 30 percent of the water (figure 1). The character of runoff reflects precipitation conditions which vary widely through the basin. The valleys west of the Cascade Range have a mild, relatively humid climate with most of the annual precipitation falling in the winter months. Inland valleys and plains east of the Cascades have a dry, semidesert climate. Interior areas of high altitude receive the greater portion of their precipitation as snow during the late fall and winter months which does not contribute to runoff until the spring of the year. Rapid melting of the accumulated snow pack, sometimes in combination with warm rains, is so general as to create severe floods cul- minating in May and June in many localities of the intermountain area east of the Cascades. The re- mainder of the season is dry. Conversely, heavy runoff in tributaries west of the Cascades occurs during the winter months as a direct result of rain- fall sometimes increased by melting of the lower snow fields. In the subbasins west of the Cascades the relatively small land area upon which a rela- tively large amount of water falls, and the high |
