Human population growth and climate change are causing biological habitats to change at a rapid pace, amplifying concerns that some species will not be able to adapt quickly enough to avoid extinction. Chinook salmon in the Pacific Northwest are particularly vulnerable to the cascade of environmental stressors associated with climate change, especially during the initial freshwater phase of their lifecycle. Early life history trajectory for Chinook salmon is directly related to environmental variation (temperature, photoperiod, flow, food availability) which can shift or alter phenotypic traits expressed by a population, such as timing of juvenile migration downstream (smoltification) and age of male maturation. For example, Fall Chinook salmon in the upper Columbia River Basin generally undergo smoltification as sub-yearlings, however some wild and hatchery reared Fall Chinook have been found to express a successful yearling smolt life history by overwintering in large reservoirs above dams. Some research suggests that this shift to reservoir type yearling smolts represents evolution, an actual genetic change due to the increase in fitness and better downstream survival of this life history type. Alternatively, the shift in life history could simply be a phenotypically plastic response that optimizes life history for the current environmental conditions. To explore this, we raised hatchery-origin Fall Chinook salmon from the Umatilla River (Oregon, USA) in a controlled laboratory experiment designed to describe and define phenotypic plasticity in life history pathways. Fish were split into 3 separate photoperiod groups at ponding: Winter Solstice, Early Spring, and Late Spring, corresponding to the seasonal range of emergence timing within this population. In addition, each photoperiod treatment was split into 2 feeding treatments with replicate tanks to simulate high or low growth. We collected data on fish growth rate, size, maturation status, and seawater survival/adaptability. Our results demonstrate that the early life history of these fish is quite plastic, with smolting occurring at different seasons, sizes and ages depending on environmental conditions. The resulting adaptable life history portfolio may provide this population of Fall Chinook salmon with some baseline capacity to cope with future environmental change.