Mill Creek is the largest watershed within the Upper Yellowstone River Sub-basin with 45 stream miles occupied by native Yellowstone Cutthroat Trout.  Non-native brook and rainbow trout are rapidly expanding in the watershed and threaten YCT populations in mainstem Mill Creek and its tributaries through competition and hybridization. Therefore, the Custer Gallatin National Forest (CGNF) is planning to construct a concrete fish barrier to secure the entire Upper Mill Creek sub-watershed as a stronghold for genetically pure YCT populations free from competing and hybridizing nonnative species, before it is too late. This single management action will secure 10.3 fish-bearing stream miles or 22% of the occupied YCT habitat in the Mill Creek watershed within climate shield cold-water refuge streams. The project site is located 22 miles south of Livingston, MT in the Mill Creek Watershed. The proposed barrier site is in a canyon on mainstem Mill Creek approximately 1/4 mile upstream from the Passage Creek confluence.