The Main San Gabriel Basin is a large groundwater basin replenished by stream runoff from the adjacent mountains and hills, by rainfall directly on the surface of the valley floor, subsurface inflow from the Raymond and Puente basins, and irrigation runoff. The illustration below simulates how water is fed, captured and stored below ground. Imported water from the State Water Project is also used to replenish the Main Basin, which serves as a natural storage reservoir. The surface area of the Basin is about 167 square miles and the freshwater storage capacity is estimated to be about 8.6 million acre-feet.
In addition to SGVMWD, two other municipal wholesale water districts import water to the Basin and are responsible for providing replacement water for overproduction: Upper San Gabriel Valley Municipal Water District (Upper District), and Three Valleys Municipal Water District (TVMWD).
The Main San Gabriel Basin Watermaster, a nine-person board appointed by the Los Angeles County Superior Court, administers and enforces the provisions of court judgments which established water rights and the responsibility for efficient management of the quantity and quality of the Basin’s ground water. The watermaster is charged with:
- Managing and controlling withdrawals and replenishment;
- Determining the annual operating safe yield;
- Raising replenishment revenue through assessments;
- Acquiring and spreading untreated imported water for groundwater replenishment;
- Coordinating efforts to preserve and restore the quality of the basin’s groundwater;
- Assisting with enforcement of water quality regulations in the basin;
- Collecting data from agencies on production, water quality, etc.;
- Preparing an annual report.
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