The cities of American Canyon and Napa are teaming up to help fund a study on a potential source of water — a new man-made lake in the Sierra Nevada foothills.

Napa County’s two biggest cities, along with four others elsewhere in the state, want to explore whether to build a reservoir and dam along the Bear River, in an area that straddles Yuba and Sutter counties.

The so-called Garden Bar Water and Power Project has been considered on and off since the 1970s. The project would include a hydroelectric power plant and would cost an estimated $500 million to construct, providing an estimated 55,000 acre-feet of water annually. That is about 15 times the annual need in American Canyon of 3,600 acre-feet.

An acre-foot is enough water to cover an acre of land one foot deep.

In addition to American Canyon and Napa, the other agencies investing in the study are the South Sutter Water District, the Castaic Lake Water Agency, the Palmdale Water District and the San Bernardino Water Municipal Water District.

The two local cities will pay a total of $167,000 for their share of the study, with Napa paying about $125,000 and American Canyon about $42,000, according to resolutions the city councils of Napa and American Canyon approved Tuesday.

Officials for both cities and a consultant on the Garden Bar project stressed the reservoir and dam will not be built for decades, if at all. “It’s just an idea at this point,” said Steve Brown, a consultant with Sacramento-based RMC Water and Environment Inc.

The study would evaluate the reservoir’s financial viability and whether there would be enough water to make it worthwhile, among other factors. The study will take about a year to complete, Brown said.

Obstacles include delivering the water to far-flung cities such as American Canyon and Riverside, as well as the fact that the state’s water delivery system is subject to complex litigation over water quality and impacts on endangered species.

Unlike the other cities in the county, American Canyon relies almost entirely on the North Bay Aqueduct to supply its residents and the Napa County Airport area with water. The city also buys a limited amount of water from the city of Vallejo and rural districts to meet demand. City officials want to avoid having to buy water from the city of Vallejo because that water is costly, they said.

The city of Napa uses state water from the North Bay Aqueduct, as well as supplies from Lake Hennessey and Milliken Reservoir. Like American Canyon, the city buys water from other regions when necessary.