The break, which occurred in the area of Foster Road, was affecting water service to homes and businesses in the Napa and Yountville areas, a Napa County sheriff's dispatcher said.
The break occurred in a 36-inch water transmission line from the Jamieson Canyon Water Treatment Plant. That transmission line delivers water to the entire city of Napa, according to city officials.
As of 5:40 p.m., crews were working to isolate the break and planned to make repairs through the night. However, there was no estimated time that service would be restored to affected customers, officials said.
Customers at higher elevations were expected to experience the greatest loss in pressure.
Dirt settling in the pipes was causing water at affected homes to appear brownish in color, officials said. Residents were asked not to contact 911 over the break or related changes in service.