A huge water storage reservoir proposed for the Yakima River Basin could be built but would cost more than twice as much as currently estimated, a federal study released Friday concludes.

The assessment by the Bureau of Reclamation estimates construction of the proposed Black Rock Dam would cost $3.5 billion to $4 billion. The current estimate is $1.8 billion.

If built at a site 40 miles east of Yakima, it would be the first new water storage in the Yakima Basin in more than 75 years.

“The answer is yes. It is technically viable,” Reclamation spokeswoman Diana Cross said from Boise. “That’s not the same as saying it’s feasible.”

The proposed earth and concrete dam would be nearly a mile wide and 760 feet high, creating a reservoir 9.2 miles long that would hold 1.7 million acre-feet of water. An acre-foot of water is enough to cover an acre of land to a depth of one foot.

It would be filled with water pumped from the Columbia River during high flows and sent to the reservoir through a tunnel or pipelines. Reservoir water would be channeled for use on lower Yakima Valley farms.

Whether a full-fledged feasibility study is conducted depends on how much political and economic support the project garners, Cross said.

“The (Bush) administration is trying to exercise restraint on discretionary funding. This project is not even in the budget for next year to continue the study,” she said. “It costs money to conduct a full feasibility study. We would only take this forward if it appears to be the alternative worth pursuing.”

More discussions are needed with potential cost-sharing partners, such as the Yakama Indian Nation, the Yakima Basin Storage Alliance and the state, Cross said.

The Reclamation study’s costs were paid by a $4 million congressional appropriation and $4 million from the state Department of Ecology, she said.