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Watermaster was created in 1973 by the California Superior Court of Los Angeles County to administer the Basin's adjudicated water rights and to provide a basin-wide governing body for management of water resources.

Beginning in the 1940s, the San Gabriel Valley experienced a period of rapid urbanization, which led to an increased demand for water drawn from the Main San Gabriel Basin. The ensuing rise in water consumption--along with an extended period of drought--had, by the 1950s, put the Basin into a state of overdraft, where water production from the Basin exceeded the amount that could be replaced.


As a result of the decrease in available water supply, parties downstream of the Basin became especially concerned. These downstream water users rely on the Main San Gabriel Basin for a large portion of their natural water supply, as much of it comes from the Basin by way of outflow through Whittier Narrows. Legal action was initiated on behalf of the downstream users, resulting in a court decision which requires the Upper Area (Main San Gabriel Basin) users to guarantee a source of water to the Lower Area or downstream users.


For several years the Upper San Gabriel Valley Municipal Water District (Upper District) administered and took responsibility for the Upper Area's obligations to the Lower Area. The Upper District could not, however, assert control over all producers in the Basin because the district boundaries do not encompass the entire Basin. In 1968, at the request of producers, the Upper District filed a complaint that would adjudicate water rights in the Basin and would bring all Basin producers under control of one governing body. The final result was the entry of the Main San Gabriel Basin Judgment in 1973.

In order to bring about an adjudication of water rights in the Main San Gabriel Basin, a complaint was filed in the Superior Court of the State of California (Case #924128, Upper San Gabriel Valley Municipal Water District, Plaintiff, vs. City of Alhambra, et al., Defendants). The original complaint was filed on January 2, 1968, but was amended on August 14, 1970 to embrace the additional issue of surface water diversion rights.


On January 4, 1973, after extensive negotiations, a stipulated Judgment in this case was entered.


The Judgment defined the water rights of 190 original parties to the legal action. It created a new governing body (the Main San Gabriel Basin Watermaster), and described a program for management of water in the Basin. Since the Judgment was originally entered, there have been subsequent amendments to it that extend and clarify Watermaster's role.


(The Main San Gabriel Basin is one of several adjudicated groundwater basins in California. The Association of Ground Water Agencies maintains a list of all the adjudicated basins and their respective Watermasters. Also, see here for more information.)



Responsibilities and Functions

 Watermaster's primary responsibilities include the following:

  • Manage and control the withdrawal and replenishment of water supplies in the Basin.

  • Determine annually the Operating Safe Yield (the amount of groundwater that can safely be extracted) for the succeeding fiscal year, and notify the pumpers of their shares thereof.

  • Acquire and spread replacement water as needed.

  • Coordinate local involvement in efforts to preserve and restore the quality of groundwater in the Basin.

  • Assist and encourage regulatory agencies to enforce water quality regulations affecting the Basin.

  • Collect production, water quality, and other relevant data from producers.

  • Prepare an annual report of Watermaster activities, including financial activities, and summary reports of pumping and diversion.


Watermaster operates under a formal set of Rules and Regulations, which spell out the procedures by which Watermaster-controlled actions are to be carried out. Under the Rules and Regulations, water producers in the Basin must obtain Watermaster approval for activities such as:

  • Constructing or modifying a well.

  • Constructing a groundwater treatment plant.

  • Increasing groundwater extraction.

  • Spreading water in the Basin.

  • Spreading and storing supplemental water under a cyclic storage agreement.


In order to fund its operation, Watermaster is authorized to levy and collect assessments from the producers based upon their amounts of production during the preceding fiscal year. These assessments are applied primarily to the purchase of replacement water and to administrative costs.




                                                                                        RULES & REGULATIONS (Revised-2013)

Area of Jurisdiction

Watermaster's area of jurisdiction is known as the relevant watershed, which overlies a portion of the upper San Gabriel River watershed. It is defined in the Judgment as the watershed area within which water rights are adjudicated. You may view a map of the relevant watershed boundary. A written legal description of the relevant watershed boundary is also available.

Mission Statement

"The Main San Gabriel Basin Watermaster, a nine-person board appointed by the Los Angeles County Superior Court, administers and enforces the provisions of the Judgment which established water rights and the responsibility for efficient management of the quantity and quality of the Basin's ground water."



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