Historical Changes in River Discharge in the Lower Columbia River: Impacts on River Stage, Tidal Range and Salmonid Habitat

Juvenile salmonid access to favorable shallow water habitat in a tidal river and estuary is a function of river stage, tidal range, and the system's distribution of bed elevation. In this study, we investigate how historical changes in Columbia River discharge, due to climatic variability and flow regulation, affect river stage, tidal range, and salmonid access to shallow-water habitat in the lower Columbia River. Tidal height data collected (1980-2001) at gauging stations below Bonneville Dam (230 km from the ocean) are filtered to retrieve time-series of fluctuations in river stage, diurnal and semi-diurnal amplitude, overtides and tidal range. It is necessary to design a new filter bank, optimized with respect to time-frequency resolution, in order to capture the relevant physics of the non-stationary riverine-tidal processes. Tidal range and tidal amplitudes of the major tidal constituents are shown to have an approximately negative-exponential dependence on upriver distance and the square root of river flow. The river stage varies linearly with river flow to the first order. These results are consistent with solutions of the linearized St. Venant equations. Further, the results imply the division of the Lower Columbia River with respect to tidal damping and stage-flow dependency into fluvial and estuarine regimes. Stage and tidal range are more strongly related to river discharge in the fluvial than in the estuarine regime. The interface between the two regimes lies in the energy minimum region, roughly 50 km from the ocean, where dissipation is weak and about equally divided between tides and river flow. With these results and hypsometeric data, scenarios of historical flow variability and their impacts on shallow-water habitat access are reconstructed. We compare: a) tides during years with low-flow to those during years having strong freshets, and b) tides under highly regulated flows with those that would have prevailed without regulation. Because tides are strongly damped by high river flows, tides were historically much smaller during the downstream migration of juvenile salmonids than is the case at present. Overall, the historic decrease in spring river stages has not compensated by the increased tidal range. Modern flow regulation coupled with diking prevent overbank flow and access to previously available habitat.