The Main San Gabriel Basin Watermaster, a nine-person board appointed by the California Superior Court of Los Angeles County, administers and enforces the provisions of the 1973 Main San Gabriel Basin Judgment, which established water rights and the responsibility for efficient management of the quantity and quality of the Basin's groundwater. It provides a basin-wide governing body for management of water resources.

The Main San Gabriel Basin underlies most of the San Gabriel Valley. The hydrologic basin or watershed coincides with a portion of the upper San Gabriel River watershed. Surface area of the groundwater basin is approximately 167 square miles. The fresh water storage capacity of the basin is estimated to be about 8.6 million acre-feet.