Impacts from human activities during the last 150 years have significantly affected essential estuary habitat. As a result of construction of hydroelectric and irrigation diversion dams, lower river and estuary habitat has been degraded, with changes in basin sedimentology, hydraulic geometry, and temperature. Although mean annual discharge has not substantially changed since 1878, Simenstad et al. (1990) estimate that the peak annual discharge has decreased from 17,500 to 12,000 cubic meters per second. The greatest change has occurred in the past 30 years, with significant changes in estuary circulatory patterns, flushing, and bathymetry.
Damping of peak and sustained flood flows by hydrosystem operations has not only resulted in the loss of critical estuarine habitat, but also has altered the trophic structure that supports salmonid production, and enhanced the accumulation of pollutants from the entire Basin in estuarine sediments (Sherwood et al. 1990).
Changes in the littoral ecology of the Columbia River estuary have been drastic. Emergent plant production has been decreased by 82% and benthic macro algae by 15% (Sherwood et al. 1990). The large blooms of phytoplankton from main stem reservoirs are washed into the estuary, where they alter food weds to the benefit of such exotic species as American shad, and to the disadvantage of anadromous salmon (Bottom and Jones 1990). Prior to the construction of the dams, organic debris, ranging in size from fine particulates to old-growth logs, washed down the river and were carried to the estuary and deposited in the ocean. Estuarine turbidity has decreased dramatically, and this has allowed for increased predation on juvenile salmon (Junge and Oakley 1966; Bottom and Jones 1990). Petts (1980) and Sherwood et al. (1990) suggest that Columbia River estuarine ecological communities may be experiencing a considerable “lag time” in adapting to rapid and profound physical changes. Because of the magnitude and rapidity of physical changes in the Columbia River estuary, ecological communities may be unable to fully adjust until conditions have stabilized.
Increases in human activity in the Pacific Ocean (Table 3.6) result in reduced populations of all species.