Constructed in 1952 by the U.S. Bureau of Reclamation to provide hydropower, recreation and flood control benefits, Hungry Horse Dam regulates the South Fork of the Flathead River located in western Montana within the upper Columbia River basin. This assessment involved the calculation and development of a metric, index of fluvial floodplain alteration (IFFA), that was used to characterize the effects of dam operations on floodplain processes. An existing hydraulic model of the Flathead River was adapted to meet project needs. Hydrologic analysis, hydraulic modeling, inundation mapping and tabulation of floodplain alteration scores were presented in a report. The Flathead River IFFA Analysis is one of several collaborative efforts between the Kootenai Tribe of Idaho and Montana Fish, Wildlife and Parks to assess floodplain ecosystem impacts associated with dam operations. The analysis has been applied to both the Flathead River and Kootenai River in northwestern Montana and northern Idaho. The analysis provides a scientific approach for linking mitigation needs to ecosystem losses. This study was funded by Bonneville Power Administration under the Northwest Power and Conservation Council’s Fish and Wildlife Program.