Environmental Flow Development: Geomorphic Considerations from the McKenzie River, Oregon

Throughout history, rivers have been altered by humans for use as a consumptive or agricultural resource. Today they are increasingly recognized for their ecological and recreational values. This has led to efforts in improving dam management through environmental flows in an attempt to mimic pre-regulated flows. In most cases, they are determined using hydrologic data, often with achieving a specific ecological aim in mind, such as improved salmonid habitat in rivers of the Pacific Northwest. However, channel characteristics, and grain size and mobility, influence most aquatic species’ habitats and are affected by flow. Important criteria for environmental flows in salmon restoration include flows that will mobilize and clean spawning gravels, and flow-sediment supply combinations that will recreate or maintain channel morphology and bed characteristics within identified salmon habitat ranges. Understanding sediment size distributions and mobility on a river is instrumental in defining environmental flows. This article discusses the need for greater consideration of geomorphology in the development of environmental flows through a case study on the McKenzie River in Oregon, presenting an evaluation of the sediment distribution and the critical discharge needed to move the median (D50) grain size on gravel bars located near three controls of flow on the river. Using field data, hydraulic modeling, and stream flow records to discuss the results, this article highlights the importance of including geomorphic and sediment characteristics in considerations for defining environmental flows and implementing them with an adaptive-management approach.