Winter storms are fattening up San Luis Reservoir, and that’s suddenly bad news for Napa’s rainy‑day water stash. The city has quietly banked carryover water there as a backup for dry years. But when San Luis gets close to full, the system’s priorities shift and those savings can shrink or vanish, leaving Napa on edge as the storms keep rolling in.
Where Napa’s Numbers Stand
According to the Napa Valley Register, Napa closed out 2025 with about 7,000 acre‑feet of carryover water sitting in San Luis Reservoir and expects that total to climb to roughly 14,000 acre‑feet when the new year starts. To hedge against losing some of that stockpile if the reservoir fills, the City Council signed off on selling up to 4,550 acre‑feet of the banked supply to Central Valley water districts for about $1.7 million. Neighboring American Canyon approved its own transfer of 1,075 acre‑feet for roughly $403,000.
City officials point out that Napa uses about 13,000 acre‑feet of water a year, so watching any of that stored supply evaporate on paper would be a serious hit to the city’s drought cushion.
How Carryover Storage Works
San Luis is an off‑stream reservoir where water pumped from the Sacramento‑San Joaquin Delta is stored for contractors in both the State Water Project and the federal Central Valley Project. The Department of Water Resources explains that this so‑called carryover water is helpful but comes with strings attached. When San Luis fills, contractors’ rescheduling rules allow that carryover to be trimmed or wiped out based on calculated “foregone pumping” while the reservoir is at capacity. That quirk is why water managers treat banked supplies as a useful but inherently shaky form of insurance.
Officials Sound the Alarm
As reported by the Napa Valley Register, Utilities Director Joy Eldredge told local officials the city likes to have some carryover water as a hedge. That hedge is now looking a bit flimsy. The Register noted that the state pumped Delta water into San Luis this fall to boost operational flexibility, a move that could put Napa’s banked water at risk if the reservoir pushes up against its limit.
Napa County Flood Control staffer Christopher Silke told the flood board that the city faces a real possibility of losing part or even all of its carryover supply, adding that he will return in February with an update once there is more clarity on winter conditions.
Why the City Sold Some of the Reserve
Offloading a slice of the carryover is essentially Napa’s way of cashing out before the house changes the rules. By selling some of the banked water, the city locks in revenue instead of watching that water get automatically reallocated if San Luis fills up. Officials framed the transfers as a balancing act: convert some of the risky reserve into money while still hanging on to a smaller emergency cushion in storage…