Assessment of Fluctuating Reservoir Elevations Using Hydraulic Models and Impacts on Larval Pacific Lamprey Rearing Habitat in the Bonneville Pool

A depth-averaged numerical hydraulic model was used to quantify the extent of dewatered sediments near the delta regions of major tributaries in lower Columbia River.  Changes in inundation were evaluated at four different forebay elevations at Bonneville Dam which included minimum operating pool (MOP),  MOP +1ft, MOP+2ft, and high pool.  The dewatered regions were calculated by importing modeled data into a geographic information system and calculating the change in inundated area.  To determine inundation changes, exceedance discharges were calculated from a ten-year period (2004–2013) from June 1 - November 30.  The 10%, 50% (median), and 90% exceedance flows were 291.4, 135.7, and 97.4 kcfs, respectively.  Changes in forebay elevation at Bonneville Dam, and the resulting impact on total dewatered regions were greater at river flow of 97 kcfs and showed greatest variation at the White Salmon/Hood River deltas.  The unsteady model output showed that water-surface elevation in the reservoir closely followed rapid changes of 1–2 ft in forebay elevation at the dam.  A 2.5-ft variation in water-surface elevation occurred during a 2-week period in February in 2002; similarly, a 3.7-ft change occurred in 2014 during the same period.  In both events, the duration of change was highly variable.