Will We Need to Change the Rules: The Implications of Climate Change for Dam Operations in Oregon's McKenzie River Basin

Dams and reservoirs are common components of many water resource systems, but their operation may be susceptible to climate change impacts due to structural constraints and limited flexibility of operational policies. Federal policy calls for dams to be multipurpose projects that allocate reservoir storage volume and outflow to various uses throughout the year. In order to balance uses and ensure that authorized purposes are met, water control plans are developed at the time of construction using historical hydrological data. Because these plans remain in place unless major structural or authorization changes are mandated, operational policies must be flexible enough to provide the authorized services of the project throughout the variety of climatic conditions over the dam's lifetime. While this implies a more-or-less stationary flow regime, climate change is predicted to have significant effects on streamflow, motivating analysis of the magnitude of effects of these changes on reservoir operations and performance. In particular, we ask whether predicted changes may require changing current operational rules, or whether streamflow objectives for both flood control and summer flow augmentation can continue to be met. We employ a two-part modeling approach to assess the risk of operational performance failures by examining a case study of Cougar Dam, a US Army Corps of Engineers multipurpose flood control dam in Oregon, USA. Synthetic streamflow sequences for historical and future climate were generated using a stochastic model that integrates output from a physically based hydrologic model together with forcing from downscaled global climate data. The synthetic streamflow sequences are the basis for simulating reservoir operations using a simple operations model. We compare past reservoir performance to measured historical elevation and flow data to validate the model. Comparing past and future simulations reveals potential effects of climate change, identifies circumstances resulting in performance failures, and allows us to quantify the likelihood of those conditions.