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.

Monitoring

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.

Resources

• Division of Safety of Dams Inspection Reports
• Division of Safety of Dams Innundation Maps

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.