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About Our Water System

A dam and its reflection on a reservoir

Chartered in 1912, Marin Water is California’s first and the oldest municipal water district. We are proud to provide locally sourced water to more than 191,000 community members in central and southern Marin County.

About 75% of our water supply originates from rainfall on 22,000 acres of our protected Mt. Tamalpais Watershed and in the grassy hills of west Marin. Rainfall from the watershed flows into the District’s seven reservoirs. The water is treated at our treatment plants and then travels through our extensive distribution system, including 908 miles of pipes, 130 storage tanks and 97 pump stations before making its way to your home or business. The District also supplements its water supply with water from the Sonoma County Water Agency, which comes from the Russian River system in Sonoma County. The Russian River water supply originates from rainfall that flows into Lake Sonoma and Lake Mendocino, and it is naturally filtered through 80 feet of sand beds adjacent to the river. It then goes through a treatment process and is blended with the District’s reservoir water within its distribution system.


Learn more about our facilities and dam safety program:

Marin Water maintains seven reservoirs – five in the Mount Tamalpais Watershed and two others in the hills of west Marin County. The oldest, Lake Lagunitas, was built in 1872, before the District was even chartered. The District’s only concrete arch dam, at Alpine Lake, was built in 1919 using the bond funding approved by voters who originally agreed to establish the District.

Six others were built, and some expanded, in the decades that followed, with the last significant modification to the District’s reservoirs coming in 1985. Together, these reservoirs are capable of holding 79,566 acre-feet of water – enough for three years’ worth of drinking water for the residents in Marin Water’s service area. Alpine, Bon Tempe, Kent, Lagunitas, and Phoenix Lake are located on the north slope of Mt. Tam. The other two – Nicasio and Soulajule – are outside the District’s service area in western Marin County.

Alpine Lake
  • Type: arched concrete dam
  • Year built: 1919, raised in 1923 and 1941
  • Storage Capacity: 8,891 acre-feet
Bon Tempe Lake
  • Type: earth-fill dam
  • Year built: 1948
  • Storage Capacity: 4,017 acre-feet
Kent Lake
  • Type: earth-fill dam
  • Year built: 1953, enlarged in 1982
  • Storage Capacity: 32,895 acre-feet 
Lake Lagunitas
  • Type: earth-fill dam
  • Year built: 1872
  • Storage Capacity: 350 acre-feet
Phoenix Lake
  • Type: earth-fill dam
  • Year built: 1905,modified in 1968 and 1985
  • Storage Capacity: 411 acre-feet
Nicasio Reservoir
  • Type: earth-fill dam
  • Year built: 1960
  • Storage Capacity: 22,340 acre-feet
Soulajule Reservoir
  • Type: earth-fill dam
  • Year built: 1979
  • Storage Capacity: 10,572 acre-feet

Every day, water from the reservoirs is pumped to one of the District’s treatment plants before passing through pump stations, storage tanks and a portion of Marin Water’s massive, 908-mile pipeline network en route to one of 61,900 service connections. 

We have a comprehensive Dam Safety Program to ensure all of our dams and spillways are safe and functioning properly. This program includes three main components: monitoring, routine inspection and maintenance, and emergency preparedness and response planning. The district also works closely with state and federal regulators and local emergency response partners to ensure public safety.


Marin Water continuously monitors conditions at the dams and spillways to ensure they are functioning as intended. Instruments placed on and in the dams monitor, and record parameters such as water seepage, dam movement and water pressure.

The engineering technicians also perform monthly visual inspections of each dam. In addition, our rangers tour the dams frequently—often daily during rainstorms—and live onsite at Phoenix, Alpine, and Soulajule dams. Other routine visits are made by maintenance and water quality lab personnel.

Inspections and Maintenance

Through ongoing maintenance, we are able to protect the dams from deterioration and prolong their lifespans. Marin Water routinely inspects the condition of each dam and spillway, and provides an annual instrumentation report to the Division of Safety of Dams (DSOD) under the California Department of Water Resources. In addition, Marin Water jointly inspects each dam with DSOD every year.

Emergency Preparedness and Response

At the core of our Dam Safety Program emergency response and preparedness is our post-earthquake dam evaluation team. After significant earthquakes, trained personnel inspect the dams for any signs of damage, and the dams’ monuments are surveyed by the district surveyor to check for any vertical or horizontal movement.

In addition, Marin Water works with other agencies and public safety personnel on emergency action planning and training exercises. Our Operations Center is staffed 24/7 year-round, so we are able to activate our Emergency Operations Center (EOC) at any time to coordinate the actions of Marin Water first responders.

We maintain Emergency Action Plans for each of our dams in the unlikely event of a dam emergency. These plans include notification procedures and contacts with the California Office of Emergency Service to assist first responders in case of an emergency. Included in these plans are inundation maps that have been submitted to DSOD.

Inundation maps show the area downstream of each dam that could be inundated by flooding from the unlikely event of a dam failure. These maps are essential for emergency response planning as they assist in determining which downstream areas of a dam may be impacted from a dam failure so that first responders can focus their efforts.


The District’s three water treatment facilities boast a combined production capacity of 71 million gallons per day. Together, the plants have treated an average of 25,000 acre-feet of water per year during the past five years of operations. That’s nearly 8.2 billion gallons of clean drinking water, and Marin Water works to ensure that water meets all state and federal standards via a battery of more than 115,000 tests annually.

San Geronimo Treatment Plant
  • Year built: 1961
  • Capacity: 35 million gallons per day
Bon Tempe Treatment Plant
  • Year built: 1959
  • Capacity: 20 million gallons per day
Ignacio Pump Station*
  • Year built: 1975
  • Capacity: 16 million gallons per day

* At Ignacio, the potable water purchased from the Sonoma County Water Agency is treated before being blended with water in the rest of the system.