The Columbia River estuary serves as an important migration, rearing, and transition environment for juvenile anadromous salmon before they enter the sea. Historical modifications to the estuary, including diking of peripheral wetland and floodplain habitats and regulation of river flows by dams, have reduced salmonid access to shallow rearing habitats and may have eliminated sources of macrodetritus that fuel the estuary's food webs. Although emerging evidence suggests that these estuarine changes are limiting juvenile salmon production and life history diversity, lack of information about historic and modern habitat conditions in the estuary, or the ecological consequences of habitat change, undermine existing salmon recovery efforts of the Columbia Basin Fish and Wildlife Program. We propose to continue a BPA-funded study to address specific information needs identified in an interdisciplinary assessment of the hydroelectric system's impacts on estuarine habitat opportunity and capacity to support diverse life histories of juvenile salmon. The primary elements of this continuing proposal include: (1) retrospective analyses to reconstruct historic bathymetric features and assess effects of climate and river flow on the extent and distribution of shallow water, wetland and tidal-floodplain habitats; (2) computer simulations using a 3-dimensional numerical model to evaluate the sensitivity of salmon rearing opportunities to various historical modifications affecting the estuary (including channel changes, flow regulation, and diking of tidal wetlands and floodplains); (3) observational studies of present and historic food web sources supporting selected life histories of juvenile salmon as determined by stable isotope, microchemistry, and parasitology techniques; and (4) experimental studies in Grays River in collaboration with Columbia River Estuary Study Taskforce (CREST) and the Columbia Land Trust (CLT) to assess effects of multiple tidal wetland restoration projects on various life histories of juvenile salmon and to compare responses to observed habitat-use patterns in the mainstem estuary. From the above observations, experiments, and additional modeling simulations we will (5) examine effects of alternative flow-management and habitat-restoration scenarios on habitat opportunity and the estuary's productive capacity for juvenile salmon.