Modeling water temperature response to dam operations and water management in Green Peter and Foster Lakes and the South Santiam River, Oregon

Abstract

Significant Findings Green Peter and Foster Dams have altered natural seasonal temperature patterns in the South and Middle Santiam Rivers of the Willamette River Basin in northwestern Oregon. Cold-water releases from Green Peter Dam, upstream of Foster Lake, contribute to the cool-water conditions at Foster Dam. In summer, unseasonably cold water typically is discharged from Foster Dam into the Foster Dam fish ladder, which may be one factor contributing to the low numbers of upstream migrating Chinook salmon ( Oncorhynchus tshawytscha ) that enter the fish ladder. The U.S. Army Corps of Engineers is leading efforts to improve conditions for Chinook salmon upstream and downstream of these dams by considering structural alterations to Foster Dam and by exploring changes to the way the dams are operated. The U.S. Geological Survey assisted the U.S. Army Corps of Engineers by using previously calibrated numerical models of flow and water quality for Green Peter and Foster Lakes and for the South Santiam River downstream of Foster Dam. These two-dimensional hydrodynamic and water-quality (CE-QUAL-W2) models were used to test scenarios of altered dam operations and alternate water-management strategies. Results of these scenarios provide information and insights into how the mixing and thermal characteristics of the lakes are affected by dam operations, how the mixing and timing of upstream source waters reaching Foster Dam are affected by dam operations, how river and fish-ladder temperature targets might be achieved, and how quickly (or slowly) such changes in the lakes and downstream river reaches occur, relative to typical unmodified operations at Green Peter and Foster Dams.