Spillway at Conn Creek Dam, Lake Hennessey

The Conn Creek Dam spillway at Lake Hennessey kicked up a giant plume from a major rainstorm in January. The dam, which feeds Napa’s main city reservoir, received a “fair” rating, second highest of four rungs, in a statewide survey released last week by the California Division of Safety of Dams.

J.L. Sousa, Register file photo

Evaluators with the state Division of Safety of Dams listed Conn Creek Dam at Lake Hennessey in “fair” condition in the report, which the agency published Friday. The 125-foot-tall earthwork structure guards a 31,000-acre-foot reservoir that has formed Napa’s main year-round water source since its completion seven decades ago.

A dam rated “satisfactory,” the highest of four rungs, has no current or potential deficiencies and is expected to hold up under all water loading and seismic conditions, according to the report.

By contrast, a dam in “fair” condition is considered adequate for a normal water load but may be vulnerable to “rare or extreme hydrologic and/or seismic events. Such a structure also may be downgraded if it has a longstanding deficiency not being addressed quickly, or if operating the reservoir at a lower level does not reduce that weakness,” authors said.

Conn Creek Dam also was rated as having an “extremely high” hazard for downstream damage in case of a structural failure, although the report added that downstream risk is not related to a dam’s condition.

The St. Helena’s lower reservoir dam, a 44-foot earthwork built in 1878, also received the state’s second-best “fair” rating. This dam is used to impound non-drinkable water that St. Helena sells to agricultural customers.