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. 

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