High Quality Content by WIKIPEDIA articles! The Plains and Sierra Miwok (the Miwok of the Sacramento Valley and the Sierra Mountains), were the largest group of Miwok Native American people. They lived in Northern California on the western slope of the Sierra Nevada Mountains between the Fresno and Cosumnes Rivers and also in the "Central Valley" in the north portion of the Delta area, where the Cosumnes, Mokelumne, and Sacramento rivers converge.