American Canyon’s northern borders could move further north during the next two decades, but only a tad, under an agreement reached with Napa County after months of difficult negotiations.

In exchange for the right to annex more than 300 acres of county land, American Canyon will agree to put any change to its ultimate boundary line to a popular vote of the city. The city will continue to serve the areas around Napa County Airport with water, according to the agreement, which expires in 2030.

“I think this is a significant step forward,” Napa County Supervisor Bill Dodd said Tuesday.

Both the Napa County Board of Supervisors and the American Canyon City Council are expected to ratify the agreement in early July. The Board of Supervisors will meet July 1. The American Canyon City Council was scheduled to discuss the deal Tuesday evening.

Under the agreement, Napa County agreed to build by 2018 a bridge for cars, bikes and pedestrians over Fagan Creek, a stream north of American Canyon’s current city line. The project, estimated at $7.8 million, also entails the construction of a section of the Devlin Road extension.

American Canyon wants Devlin Road as an alternate route to congestion-plagued Highway 29.

The city’s eastern boundaries will remain within the 15 percent slope line and include Town Center, a residential and commercial development in the planning stage, as well as the future American Canyon High School site off American Canyon Road.

The agreement has teeth, said Dodd, referring to the “poison pill” that would force the jurisdiction that breaks the agreement to repay tax revenues to the other.

“We want it to last,” said Dodd, who praised the efforts of American Canyon City Councilmen Don Callison and Ed West for pursuing the negotiations with Napa County.

Under the agreement, the 300 acres will remain industrial.

Supervisor Diane Dillon on Friday said the agreement brings certainty to the issues being negotiated and avoids time-consuming adversarial discussions.

Mayor Leon Garcia, like Dillon, also said the deal brings certainty.

“We are clear about the expectations of each other,” Garcia said Friday after a countywide meeting on growth in Napa County.

City Manager Rich Ramirez last week said the deal will bring economic sustainability, referring to the industrial land slated for annexation and development.

Volunteers with Impact 94503, a grass-roots organization in American Canyon that supports the development of the Town Center project, will collect signatures from city voters to ensure the new ultimate boundary line becomes effective this year, said Mark Joseph, the former city manager.

The Hess property, vineyards on the east side of Highway 29, will remain outside the city’s ultimate boundary line. Under the recently-approved Napa County general plan, the 240 acres are zoned agricultural.