Photo credit: Ken Popper/TNC

The Army Corps of Engineers (Corps) operates a system of 13 dams in the Willamette Basin, all of which are located on major tributaries. The dams provide a range of human benefits including flood risk management, hydropower, irrigation, navigation and recreation. However, operation of these dams has changed the volume and timing of water flow in the river and tributaries, resulting in reduced peak flows, lower spring flows, increased summer low flows, and infrequent bankfull events. Alterations to the natural flow regime affect the health and viability of the freshwater ecosystems and the aquatic and terrestrial species and communities they sustain. 

To address these impacts, the Corps and The Nature Conservancy (the Conservancy) are working together to determine how management of the Willamette dams can be modified for environmental benefits while maintaining important human uses such as flood damage reduction, hydropower generation and recreation. The Corps and the Conservancy worked with a group of local experts and stakeholders to develop environmental flow recommendations for each of the major tributaries with Corps dams. This information was integrated in the Corps’ reservoir simulation model to evaluate alternative operational strategies for implementing the environmental flows basin-wide and to assess the effects on other project purposes including flood risk management and hydropower. The results provided operational strategies that can be used to maximize opportunities for achieving basin-wide environmental flows without adversely impacting other project purposes. 

Initial implementation and monitoring of environmental flow releases began in 2008 and has continued through the present time. The goal of initial environmental flow implementation is to evaluate and test the process, as well as to identify physical, biological and institutional opportunities and constraints. Currently the Corps and the Conservancy are working with the U.S. Geological Survey to develop a comprehensive monitoring framework to assess basin-wide implementation of environmental flow recommendations and inform an ongoing adaptively managed environmental flow program.

Preliminary results indicate that the environmental flow releases are providing ecological benefits.  For example, data from a series of water level recorders installed along the river and floodplain are documenting side channel and floodplain connectivity during environmental flow releases.  Recent studies of changes in channel and floodplain features from 2005 (pre-implementation) to 2014 are demonstrating benefits to channel complexity and aquatic and riparian habitat from environmental flow releases.