EVALUATING ANTHROPOGENIC IMPACTS ON SALMONID HABITAT WITH A TWO-DIMENSIONAL HYDRAULIC MODEL, MIDDLE FORK JOHN DAY RIVER, OREGON

A broad-scale multi-disciplinary channel assessment of the Middle Fork John Day River, Grant County, Oregon, was performed in support of the recovery of listed salmonid species under the Endangered Species Act. The assessment recognized the high potential to reestablish physical processes and address limiting factors of the salmonid species within a four-mile reach in the Forrest Conservation Area, a property owned and operated by the Confederated Tribes of the Warm Springs Reservation of Oregon. A need for a more detailed hydraulic model and geomorphic investigation on this reach was identified to explore the impacts of anthropogenic activities on channel processes. Over the past 150 years, anthropogenic activities on the Forrest Conservation Area have disconnected the channel from more than fifty percent of the floodplain area primarily due to a historical railroad grade. Flow is currently conveyed through an entrenched artificial channel that has resulted in an increased channel slope and reduced length. A two-dimensional hydraulic model was used to quantify the extent to which constructed features and human activities limit spawning, rearing, and holding habitat for listed salmonid species. Two topographic scenarios were simulated using Bureau of Reclamation’s SRH-2D hydraulic model to define the existing conditions of the reach and to evaluate potential increases in habitat through the removal of the historical railroad grade. The first topographic scenario was developed based on conditions that exist within the reach today, while the second scenario represented the topography of the reach if the railroad grade was completely removed and flow was conveyed through its historical channel. Topographic data were suitable to model discharges at or exceeding bankfull discharge. Distributions of depth, velocity, and shear stress were examined to determine how and where changes would occur between the two modeled scenarios for select discharges between the 2-year and 100-year recurrence intervals. Using results from the model, several habitat indicators, including side channel access, floodplain connectivity, and high-quality high-flow habitat, were compared between existing conditions and removed railroad conditions to characterize potential improvements to habitat. Results from the modeling effort indicate that completely removing the historical railroad grade may not be necessary. The model can be further applied to identify whether partial removal or strategic breaching of the railroad grade would be equally beneficial biologically to complete removal. Information from the modeling will be linked to geomorphic investigations and can be integrated with ecological processes to assist managers, partner agencies, and design committees in focusing resources towards implementation of sustainable rehabilitation strategies that gain the greatest biological benefit.