Human activity in the Central Valley predates the formation of the Delta six thousand years ago. At the time of contact by Europeans, roughly half of the entire native population of California lived in the Sacramento and San Joaquin Valleys, already altering the landscape for their own use. These tribes, like the Miwok and Yukut, while not practicing agriculture as we know it, pruned vegetation to stimulate growth, used nets, harpoons and hook-and-line to trap fish and birds, and used fire to manage desirable food plants and wildlife. It is estimated that Native Americans may have harvested as much or more salmon annually from the rivers as did the fishermen and canneries in the late 1800s, although the populations did not suffer until the effects of hydraulic mining added new stressors to the salmon’s environment.