Impacts of Columbia River discharge on salmonid habitat: 2. Changes in shallow‐water habitat

This is the second part of an investigation that analyzes human alteration of shallow‐water habitat (SWH) available to juvenile salmonids in the tidal Lower Columbia River. Part 2 develops a one‐dimensional, subtidal river stage model that explains ∼90% of the stage variance in the tidal river. This model and the tidal model developed in part 1 [ Kukulka and Jay, 2003 ] uncouple the nonlinear interaction of river tides and river stage by referring both to external forcing by river discharge, ocean tides, and atmospheric pressure. Applying the two models, daily high‐water levels were predicted for a reach from rkm‐50 to rkm‐90 during 1974 to 1998, the period of contemporary management. Predicted water levels were related to the bathymetry and topography to determine the changes in shallow‐water habitat area (SWHA) caused by flood control dikes and altered flow management. Model results suggest that diking and a >40% reduction of peak flows have reduced SWHA by ∼62% during the crucial spring freshet period during which juvenile salmon use of SWHA is maximal. Taken individually, diking and flow cycle alteration reduced spring freshet SWHA by 52% and 29%, respectively. SWHA has been both displaced to lower elevations and modified in its character because tidal range has increased. Our models of these processes are economical for the very long simulations (seasons to centuries) needed to understand historic changes and climate impacts on SWH. Through analysis of the nonlinear processes controlling surface elevation in a tidal river, we have identified some of the mechanisms that link freshwater discharge to SWH and salmonid survival.