California’s complex system of water rights took shape starting in the mid-1800s, when settlers saw the state’s water as abundant and free for the taking — a time when a Gold Rush prospector could stake claim to river flows simply by nailing a notice to a tree. Today, California’s oldest and most senior water rights — called riparian and pre-1914 rights — have been passed along to thousands of agricultural landowners, irrigation districts and urban water suppliers that claim control of roughly one-third of the water that is diverted from the state’s rivers and streams.