The daunting question faced for years now by a small farming valley in Mendocino County is where it will turn to for irrigation water once the Lake County dam that impounds its main source, Lake Pillsbury, comes down.

Owner Pacific Gas & Electric Co. is seeking federal approval to decommission the dam and the connected hydropower project that for more than a century has funneled Eel River flows through a mountain tunnel into canals that feed Potter Valley, and eventually emptying into the Russian River, boosting its supplies for downstream cities and farms.

As expected, once Scott Dam is gone along with its downstream waterworks — a removal project PG&E estimates to cost $500 million — Potter Valley growers, ranchers and residents worry they could be left high and dry.

Officials in Mendocino and Sonoma counties have been working on backup storage options to avoid that scenario and sustain the valley’s $35 million agricultural output, the lifeblood of its rural community.

At a Monday meeting in Ukiah, they unveiled some of the early concepts, little more at this stage than just “cartoon drawings,” as Tom Johnson, engineering consultant with the Mendocino County Inland Water and Power Commission, or IWPC, acknowledged at the meeting.

The federal decommissioning process is not a fast one, and any dam removal project, should it be approved, is likely years away.

But building new water storage in California is not easy or cheap.

“We need to figure out storage before Scott Dam goes away and before we have little water,” said Scott Shapiro, legal counsel with the water and power commission, the joint powers entity that oversees water use and quality in the Eel and Russian River watersheds.

The infrastructure at issue actually consists of two dams — Scott and the smaller Cape Horn Dam at the foot of a holding reservoir downstream — plus the idled 117-year-old powerhouse fed by the diversion tunnel between the two watersheds. The shorthand name for it all is the Potter Valley Project, and PG&E began taking steps in 2019 to abandon it.

Under a historic agreement reached early this year involving tribes, local governments, environmental interests and water entities, both dams are set to come down — in what would be the nation’s next big dam removal project, freeing up the headwaters of California’s third longest river to help revive its troubled salmon and steelhead trout runs.