Green Peter Dam on the Middle Santiam River and the downstream Foster Dam on the South Santiam River in Oregon have altered natural seasonal temperature patterns in those rivers. In response, 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 and by exploring changes to the way the dams are operated. This data release includes the input and output files from a mechanistic water-quality model (CE–QUAL–W2) that was developed for Green Peter and Foster Lakes and the South Santiam River downstream of Foster Dam. The model was used to estimate how operations at Green Peter and Foster Dams affect water temperature in Foster Lake and in the South Santiam River downstream of Foster Dam, and how the dams can be operated to produce more optimal water temperatures to support fish requirements. The model results provide guidance to reservoir operators and water-resource professionals who seek to manage the South Santiam River Basin for multiple purposes, including improved conditions for endangered anadromous fish.