Wild lands surrounding two city of Napa reservoirs — Lake Hennessey and Lake Milliken — may be opened to the public for hiking, biking and horse riding.

The Napa County Regional Park and Open Space District is planning nearly six miles of trails at Lake Hennessey and 10 miles of trails at Lake Milliken, including a stretch that would be part of the Bay Area Ridge Trail.

City officials are supporting public recreation at Lake Hennessey, located 30 minutes from Napa on Highway 128, as long as trails are routed away from the reservoir that supplies water to more than 75,000 people.

The city won’t hear the district’s request for public access to the much wilder and steeper terrain at Lake Milliken on Atlas Peak Road until it can evaluate how things have gone at Hennessey.

The first trails at Lake Hennessey could be ready for public use by the summer of 2011, Dave Finigan, president of the district’s board of directors, told the Napa City Council Tuesday night.

The Lake Hennessey trails would begin at a new park, Moore Creek, a 673-acre property on Chiles-Pope Valley Road purchased by the district a year ago.

The district will need to complete an environmental study and get a county use permit before it can start to build facilities at Moore Creek. The City Council must also give the project final approval.

Phil Brun, a deputy director of public works, said he could support trails at Lake Hennessey as long as they were kept away from the dam, a water treatment plant and a field of solar collectors and the water.

Currently, the city allows small boats on the lake, but prohibits swimming and water skiing.

The city owns some 4,000 acres at Lake Hennessey. The trails would be confined to land north of the reservoir. A caretaker at Moore Creek would supervise the park.

These reservoir proposals would maximize the use of land already in the public domain for public recreation, Finigan said.

The district wants the trails at Moore Creek, Lake Hennessey and Lake Milliken to be open to hikers, equestrians and mountain bikers. The city accepts multiple uses at Hennessey, but wants to put off discussion of Milliken until later.

Once it has final approvals from the county and city, the Park and Open Space District will spend $50,000 to open up Moore Creek and construct one-third of a mile of trail connecting the property to a dirt road at Lake Hennessey.

Another two miles of trail would be built as money became available, Finigan said.

The district is continuing to negotiate with private property owners adjacent to Lake Milliken to buy land for an access road and staging area off of Atlas Peak Road, John Woodbury, the district’s general manager, told the council.

The district would need to raise funds to construct 10 miles of planned trails at Milliken, which is a 15-minute drive from Napa, behind Silverado Country Club. None currently exist.

The district bought the Moore Creek property for $2.8 million, using a $1.65 million grant from the state Coastal Conservancy and state park bond money.