Yellowstone National Park will design a barrier to halt invasion of nonnative trout into Yellowstone’s Soda Butte Creek, a stronghold for Yellowstone cutthroat trout that has recently been invaded by rainbow trout.  The recent rainbow trout invasion of upper Soda Butte Creek, a historic stronghold for native Yellowstone cutthroat trout (YCT) and a popular fishery, is cause for great concern. Ice Box Canyon was long believed to isolate the upper 15 miles of Soda Butte Creek from rainbow trout in the Lamar River, therefore securing it from nonnative threats. However, early in the 2000s, angler reports began to indicate that rainbow trout were present in upper Soda Butte Creek and subsequent genetic analyses confirmed their presence. Around the same time brook trout, present in an upstream tributary, exploded in the system and were immediately recognized as a serious competitive threat to cutthroat trout. Hybridization and competition of this nature threaten the persistence and genetic integrity of the entire upper Soda Butte Creek YCT population. In response to the threat, an interagency effort to mechanically remove nonnative fish from the system has been underway since 2004. Recent investigation of Ice Box Canyon indicates that it is not a barrier to upstream fish movement under all flow conditions but could be made a permanent barrier through modification of the existing cascade. This project will identify the most efficacious and cost effective means for modifying the Ice Box Canyon into a complete barrier, continue nonnative suppression, document changes in YCT genetic integrity, and develop effective strategies to ensure the future of Soda Butte Creek’s cutthroat trout. Project partners include Beartooth Alliance, Montana State University, U.S. Forest Service, Volunteer Fly Fishing Program, Yellowstone Park Foundation, and Montana Fish, Wildlife, and Parks.