Hydrological processes impact the functioning of ecosystems and influence fish population dynamics. In the Great Smoky Mountains National Park (GRSM), native brook trout (Salvelinus fontinalis) and rainbow trout (Oncorhynchus mykiss) populations have declined in some watersheds over the past two decades. Since 1986, 69 streams (369 sites) are routinely sampled by GRSM fisheries biologists, in which detailed data have been collected on fish condition (length, weight), abundance, year class strength, and biomass. To evaluate the impact of floods and droughts on trout, indicators of hydrologic alteration (IHA) were computed for 127 fish sampling sites between 1990 and 2007. The Hydrological Simulation Program - FORTRAN was used to model flows at each ungaged fish-sampling stream site. Outlets were defined for each site, and flow was calculated for the period of record. The model was calibrated by adjusting parameters including storage, infiltration, runoff, and ground water for three elevation classes (low<800m≤medium<1200m≤high) to fit flow output from model with two USGS gaging stations and one NPS gaging station. The ecohydrologic regimes, characterized using the IHA method, were compared with trout abundance and biomass at each site. Results indicated the abundance of young-of-the-year brook and rainbow trout significantly declined after extreme floods and droughts. In particular, low-flows during droughts significantly reduced recruitment for both brook and rainbow trout, which is likely due to decreased spawning habitat. Brook trout populations in larger low-elevation streams showed more stability compared to smaller headwater streams.