The varied habitats required by anadromous fish have been altered extensively since European settlement of the region, and attempts to manage these habitats have become increasingly complex. Generally, the impacts of these alterations are experienced in more than one habitat, e.g., altered flow regimes, blocked access, water quality and quantity. They begin in the tributaries and continue in the mainstem, estuary, and potentially the ocean. For example, mainstem dams and their operation have created a series of reservoirs along the mainstem, whichled to altered flows, higher temperatures, and related changes in habitat conditions in the estuary.
Destruction of habitat required by anadromous species has occurred in a variety of ways throughout the Basin. For salmon, human impact in the Columbia River Basin is often categorized as the “4 Hs”–those related to the Hydrosystem, Habitat, Hatcheries, and Harvest. Because the needs of anadromous organisms encompass habitats in different ecosystems, any break in the habitat chain becomes a problem for these species. Weakening of a given link similarly reduces the ability of these organisms to cope with environmental stress. Among the major changes addressed by this plan for restoration in the Columbia River Basin are those involving reductions in available habitat, habitat degradation, water quality impacts, mortalities related to passage, and harvest.