The operation of the series of Columbia Basin dams and storage reservoirs and the diking and development of structures for flood control has served to disconnect key productive areas from the river flow continuum. Impacts include the loss of floodplain vegetation and loss of nutrient capital essential for fish production (Sedell and Frogatt 1984). Welcomme (1979) and Ward and Stanford (1989) investigated large floodplain rivers rivers worldwide and found that the loss of floodplains was the greatest reason for the loss of productive fisheries. For example, the loss of 67% of floodplain habitat in the Missouri Basin was accompanied by a greater than 80% reduction in fish catch (Whitley and Campell 1974).
The loss of downstream organic and inorganic nutrient cycling from dam construction and operation has been profound. In reviewing ecological impacts of dams in major rivers worldwide, Petts (1987) and Regier et al. (1989) found that the accumulation of nutrients behind dams has lowered the diversity of trophic orders necessary to support higher orders of biota and that this is a continuing process over time.
The loss of nutrient cycling upstream from dam passage losses has also been profound basinwide. Dams have both reduced the widespread distribution of salmon carcasses into the mainstem and tributaries by direct means and contributed to the loss of energy reserves. The importance of salmon carcasses in providing a trophic base for subsequent year classes was documented by Cederholm et al. (1989), who also found that the carcasses were utilized by 22 species of birds and mammals.
Source of mortality to anadromous fish in mainstem areas that are not directly are not directly blocked include passage through turbines, inadequate adult and juvenile bypass systems, indirect effects of power peaking, slackwater reservoir passage, and migration delays resulting from the altered hydrologic regime. Estimates of passage mortalities through the series of mainstem dams average 15-30% per dam for juveniles and 5-10% per dam for adults (NPPC 1986; Raymond 1988; Kaczinski and Palmisano 1992). Thus, cumulative mortality through the nine-dam mainstem system may be as high as 98% for juveniles and 37-51% for adults during low-flow years (1988). Further, salmon must pass additional barriers in many of the tributaries (e.g., the Yakima River).
After 1970, with the completion of huge Canadian storage reservoirs and the construction of John Day Dam, four public utility dams in the Mid-Columbia, and three Lower Snake River dams, large and consistent declines, from about 4% to about 1%, occurred in average smolt-to-adult survival rates (Raymond 1988; Petrosky and Schaller 1992). Numerous alternatives were developed with much expense and effort to address juvenile passage — mechanical bypass, juvenile barging and trucking, and spill (Anderson 1988; Raymond 1988). Yet each of these alternatives has met with confounding drawbacks. For example, spill, if not carefully controlled, can cause nitrogen supersaturation (Ebel et al. 1975; Ebel and Raymond 1976; Ebel 1979; Weitkamp and Katz 1980). Mechanical bypass systems may divert only 8-30% of sub-yearling chinook and sockeye from turbines (Gessel et al. 1989, 1990; USACE 1993; Brege et al. 1992). Twenty years of transportation have not stemmed the steady decline of Snake River chinook stocks (congleton et al. 1985b). Adult salmon, accustomed to working with natural river hydraulics to move upstream, are confused by flows at the base of dams. This leads to delay in moving expediently through adult fishways (Muir 1957; Bjornn and Peery 1992). Further, lack of proper hydraulic cues in dam forebays can cause adults to “fall back” over the spillways and through turbines. For example, during one week, nearly 800 steelhead have been documented to fall back through McNary Dam (WDF 1991).
An additional impact related to changes in the system’s hydrograph involves travel time for migration. Whereas juvenile salmon historically took an average of 22 days to reach the estuary from the upper reaches of the Snake River, the trip today requires over 50 days. This prolongs the period during which migrants are exposed to heightened predation, disease, and increased water temperatures, and interferes with essential physiological development (Li et al. 1987; Raymond 1988). Average peak flows at the Dalles have been reduced from 400-500 kcfs to less than 200 kcfs. Concurrent impoundment of organic sediments has produced large blooms of phytoplankton (Sherwood et al. 1990), which have benefited exotic species such as shad.
Since the last of the basin’s large storage reservoirs was created in Canada less than 20 years ago, it is likely that the Columbia basin ecosystem is still in a period of ecological adjustment. Because of the complexity of the cumulative changes and the time lag in which ecosystems attempt to stabilize environmental disturbances, it is not known at this time what the final ecosystem of the Columbia River will contain with respect to trophic assemblages. However, the historical record from other major rivers in the world indicates that unless major management changes are instituted to restore riverine physical and chemical habitat processes, the Columbia River ecosystem will not likely continue to support anadromous salmon into future centuries.