In river networks, productivity increases from headwaters to larger mainstems as channels widen and receive more sunlight. At confluences of small tributaries and large mainstems, the dark, cool and unproductive habitat, and the sunlit, warm and productive habitat are adjacent to each other. In this talk, I will describe the migratory life history of a mayfly Ephemerella maculata (Ephemerellidae) that rears and emerges from the productive mainstem. Females mate, then fly into adjacent small tributaries, oviposit, die, and drift into pools to support aquatic and riparian predators in these unproductive habitats. A large field manipulative experiment has shown that the resource subsidy by the mayfly migration significantly increases the growth of juvenile steelhead trout rearing in tributaries. The habitat of juvenile steelhead trout in river networks in California is increasingly restricted to cold tributaries due to the temperature rise in the mainstem. The resource redistribution mediated by this migratory mayfly could play a critical role to support juvenile steelhead trout in the warming rivers in California.