A common goal of biological adaptation planning is to identify and prioritize locations that remain suitably cool during summer. This implicitly devalues areas that are ephemerally warm, even if they are suitable most of the year for mobile animals. Here we develop an alternative conceptual framework, the growth regime, which considers seasonal and landscape variation in physiological performance, focusing on riverine fish. Using temperature models for 14 river basins, we show that growth opportunities propagate up and down river networks on a seasonal basis, and that downstream habitats that are suboptimally warm in summer may actually provide the majority of growth potential expressed annually. We demonstrate with an agent-based simulation that shoulder-season use of warmer downstream habitats can fuel annual fish production. Our work reveals a synergy between cold and warm habitats that could be fundamental for supporting coldwater fisheries, highlighting the risk in conservation strategies that underappreciate warm habitats.