large number of small (relative to the Columbia’s flows) withdrawals.

The current pattern of withdrawals is such that they have very little effect on Columbia River flows during January, a period of low demand. By contrast, the volumes of withdrawals in July and August—a period of highest demand—have noticeable effects on Columbia River flows. Although hydrological data on Columbia River withdrawals are imperfect, the data that are available suggest that summer withdrawals in July divert roughly 16.6 percent of river flows at John Day Dam. The upper end of the range of prospective additional withdrawals considered in this study would increase that figure, raising it to roughly 21 percent. A key issue in considering the implications of prospective additional water diversions clearly is the seasonality of those diversions.

Other important changes to the river include deteriorating water quality, which has implications for Columbia River salmon, and increasing water temperatures. Water temperatures in the mainstem Columbia increased steadily during the latter part of the twentieth century. Most observers attribute this increase to the construction of dams and impoundments along the river. Other watersheds in the region that have had fewer hydrological alterations—such as Canada’s Fraser River—exhibit increases in water temperature in the absence of impoundments (the magnitude of temperature increases there, however, is smaller than in the Columbia). Prospective climate warming across the Columbia basin may thus also be contributing to this trend. Although precise cause-and-effect mechanisms are hard to define clearly, the changes in Columbia River hydrology identified in this chapter have greatly affected the basin’s salmon populations. The following chapter examines relationships between Columbia River salmon and several environmental changes and variables.