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Frequently Asked Questions


Who is the Mendocino County Inland Water & Power Commission and what do they do?

In 1996, the Mendocino County Inland Water and Power Commission (IWPC) was formed as a joint powers authority to serve as stewards of the Russian and Eel River watersheds, safeguarding the water that plays such a vital role to the region’s economic development, environmental quality, and general well-being of those who use this water.

IWPC is deeply involved in protecting the future of the Potter Valley Project (PVP) because the Project provides water to over 650,000 people from Mendocino County through Marin County by supporting agriculture, wildlife, fisheries, and many local communities.

IWPC members include:

  • County of Mendocino
  • City of Ukiah
  • Redwood Valley County Water District
  • Potter Valley Irrigation District
  • Mendocino County Russian River Flood Control and Water Conservation Improvement District

IWPC is the non-federal local sponsor along with the Army Corps of Engineers to look at the feasibility of raising Coyote Dam at Lake Mendocino for increased storage and water reliability during drought years.

Who owns the Potter Valley Project (PVP)?

PG&E has run the hydroplant since 1932 but in 2019 the company notified the Federal Energy Regulatory Commission (FERC) that they would not be applying to relicense the PVP. At that point, if no one was willing to take over the license and to keep the water flowing through the tunnel, there was concern that the water supply would be stopped altogether. This would create risk and uncertainty for the Eel and Russian River basin communities and imperiled fish.

Sonoma Water applied for a grant to collaboratively problem-solve for the future of the PVP diversion, water supply resiliency and fisheries enhancement. Part of the grant funding is being used to form a regional water forum dedicated to addressing the continued diversion of water from the PVP. Mendocino County Inland Water & Power Commission (IWPC) is part of the newly formed Russian River Water Forum that is working diligently on a solution, including developing a new owner/operator, for the PVP. This regional entity would represent water interests in the Russian/Eel River watersheds and would support continued water diversions, water supply resiliency and improved aquatic habitat.

Where does the water for the Potter Valley Project come from?
The headwaters originate in Lake and Mendocino Counties

Waters from the tributaries above Scott Dam provide the water supply for Lake Pillsbury. Below Scott Dam, waters that are released combine with tributaries that originate in Mendocino County.

Is it true that the ‘run of the river’ diversion will work because it worked fine between 1908 and 1922 before Scott Dam was built?

Before 1900, the Russian River would often go dry during the summer months. Because the Eel River only had natural flow during the rainy winters and early spring, a summertime water supply was needed to produce power at the Potter Valley Project year-round. The only way to realize a summertime water source was to capture rainfall in the winter, store it and release it to run through the powerhouse during the dry late spring, summer, and fall months. To achieve this goal, Scott Dam, forming Lake Pillsbury, was built in 1922 twelve miles upstream of the PVP Van Arsdale Reservoir. The PVP could now store water in Lake Pillsbury during the winter and release it to run its turbines year-round. Remarkably, for all the people in Potter Valley and much farther south, surface water was suddenly flowing in the Russian River in the summertime! Without Scott Dam, water flow levels would return to pre-1922 conditions. Recent droughts have only made these conditions worse.

Is it true that the loss of PVP water is fear mongering and that it isn’t about drinking water…it’s just so Ag users can have free water? How many people are dependent on this water source?

Currently, most agricultural and domestic users of the water diverted from the PVP do not pay directly for the water, though they may pay for distribution and treatment. Only Potter Valley Irrigation District (PVID) has paid for this water since PVID was formed in 1926. Part of the effort to inform users about their dependency on this water supply is to help them understand that they will need to pay enough for the water to fund the acquisition, management and maintenance of the diversion facilities.

PG&E’s decision to surrender the license could result in the elimination of this vital source of water for the Russian River basin and have significant impacts on our communities’ domestic, agriculture and fire suppression water supplies. Without water from the PVP, that serves over 650,000 people:

  • In 8 out of 10 years, Lake Mendocino will not be able to meet all the water supply needs of businesses, cities, residents, and wildlife that depend on it. In 2 out of these 8 water-short years, Lake Mendocino is predicted to go dry.
  • The volume of water in Lake Mendocino that salmon and steelhead need for spawning and rearing in the upper Russian River would be diminished and cause further declines in the fisheries.
  • Losses in the tens of millions per year to the economies of Mendocino and Sonoma counties are expected. A major concern for many is the fear of future fires and our ability to fight them if there isn’t enough water flowing during the summer months.
I heard that they were looking at the possibility of raising Coyote Dam at Lake Mendocino to allow for more storage. What has happened to that project?

IWPC is the non-federal local sponsor along with the Army Corps of Engineers to look at the feasibility of raising Coyote Dam at Lake Mendocino for increased storage and water reliability during drought years. Nothing can move forward regarding studies needed until the federal government allocates funding for the project. IWPC will need to match those dollars to move the studies forward and is looking to grant funding at the state and/or federal level and local assistance to meet this goal.

Once water is diverted from the Eel River, where does it go?

Water is diverted through a tunnel at the north end of the Russian River watershed in Potter Valley into the east branch of the Russian River. From there, the water flows downstream into Lake Mendocino and below Coyote Valley Dam in the Russian River to the ocean at Jenner.

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How much water is diverted from the Eel to the Russian River through the PVP?

On average, a small fraction, 1.8% (60,000 acre-feet) of the flow of the entire Eel River is diverted through the Potter Valley Project annually. Prior to 2006, up to 120,000 acre-feet of water, approximately 4% was diverted annually through the PVP. As a result of extensive studies undertaken to better understand the impact of the PVP on the Eel River during the relicensing and license amendment process, the amount of water diverted from the Eel River was dramatically reduced to less than half of historical transfers.

Does Sonoma County take most of the water from Lake Mendocino?

Lake Mendocino was created in 1950 for flood control and water supply. It was a joint project of the Army Corps and the Sonoma County Water Agency (SW) which secured 87 percent of the water stored in the lake for domestic, fisheries, and irrigation supplies. The Mendocino County Russian River Flood Control and Water Conservation Improvement District (RRFC) was formed to share in the costs and secured rights to 8,000 acre feet or 11% of the available water for local use. Sonoma County has since constructed Lake Sonoma on Dry Creek, and now serves some of its customers in Sonoma and northern Marin counties from Lake Sonoma. SW is required to release water from Lake Mendocino in the summer months to satisfy water right holders along the Russian River and to provide minimum instream flows for fish habitat. Without adequate water storage in Lake Mendocino, the volume of water that salmon and steelhead need for spawning and rearing in the upper Russian River will be diminished and cause further declines in the fisheries.

Sonoma Water believes a portfolio of water supply initiatives is required to protect beneficial uses in the Russian River watershed. These include:

  • Appropriate volume and timing of continued transfers through the PVP
  • Continued improvement of water use efficiency
  • Managed Aquifer Recharge
  • Conjunctive water management
  • Recycled water
  • Forecast Informed Reservoir Operations for Lake Mendocino and Lake Sonoma
  • Russian River Biological Opinion Fish Flows Project
Why do they let so much water out of Lake Mendocino during the winter months?

A Water Control Manual (WCM) is developed for each Army Corp of Engineers dam that includes flood control as an authorized purpose. The WCM guides water release decisions for the dam. A major element of a WCM is the guide curve which describes the maximum allowable reservoir storage levels to meet flood control objectives during the winter months. Water is released ahead of storms to allow for storage of the incoming water. However, the WCM for Lake Mendocino was developed in the 50’s and does not work well with our current weather patterns. Since weather forecast skills have improved in recent years, the USACE has updated their regulations to allow for more accurate use of forecasts with a new system.

This new forecasting system, Forecast Informed Reservoir Operations (FIRO), allows the Corps the discretion to hold back an additional 9,500 acre of surplus water through Feb. 15 unless another atmospheric river is forecast. The discretionary amount rises to 19,000 feet by March 1, absent any major storm activity, according to Sonoma Water’s fact sheets.

The USACE increased its target storage amount, from that prescribed by the WCM, for the early winter months up to 80,000 AF from 68,000 AF due to FIRO. See the chart below from Sonoma Water’s website (click image to enlarge):

Where can I get more information on the history of the Potter Valley Project and what is happening now?

There are several good websites for information on the PVP including:

Our Shared Water


Potter Valley Water Project History


Potter Valley Water Project Licensing
What is being done to save the water diversion at the PVP and who is involved?

IWPC is currently engaged with the Russian River Water Forum (RRWF) to collaboratively problem-solve the future of the PVP diversion, water resiliency and fisheries in the river basins. They are also working to develop informational materials for the public to educate them on their water supply, how it is being threatened and what they can do to become involved in saving it.

Substantial local, state, federal and other funding is necessary to:

  • Complete modifications to the PVP water diversion facilities to allow water to continue flowing into the Russian River basin after decommissioning.
  • Fund ongoing operation, maintenance, and administration of the PVP’s water supply facilities.
  • Develop a program for resilient and reliable water supplies in the Russian River basin.
  • Improve fish passage and habitat in the Eel River.
  • Provide enough water for effective fire fighting during the dry summer months.
How can I get involved in saving the water flow from the PVP which provides our community with water, food, and economic viability?

Stayed informed: Read the latest updates on the IWPC website under the ‘PVP Stay Informed’ tab.

Stay Informed

Sign up on the IWPC website to be alerted when you are needed to write letters of support or attend a meeting to fight for the future of the PVP water diversion:

Subscribe for Alerts

Support IWPC by donating to the efforts to save the PVP at:


Attend board meetings when the PVP is being discussed and/or read the minutes of IWPC, Mendocino County Board of Supervisors, City of Ukiah City Council, Redwood Valley County Water District, Potter Valley Irrigation District and the Russian River Flood Control District.

Be an advocate for saving the PVP by telling your family and friends about:
1. how their water supply is being threatened
2. how we will need to work together to protect it
3. and how we can improve regional water supply resiliency and enhance the health of our rivers.

Why should I care?

It is important to maintain local control of the Project to continue providing a crucial water source for the communities and environment that have developed around the water supply over the last 100+ years.

We want to ensure that we continue to have reliable water storage with year round supply.

Learn More
What are elected representatives doing about this?

Our federal, state and local elected officials are:

  • Congressman Jared Huffman
  • Senator Mike McGuire
  • Assembly Member Jim Wood
  • County Board of Supervisors
  • Local Elected Officials in cities and water districts