JUNEAU - Crews could be running a major hydroelectric dam in Alaska by 2022, but the prep work could require a big, up-front cash investment, state energy specialists said Wednesday.

Finance and design details are still on the table, they said, but rough estimates suggest a 600-megawatt dam could let utilities buy power for a little more than 6 cents per kilowatt-hour.

That's about the same price Anchorage's Chugach Electric Association pays for natural gas-fired power and generally less than Golden Valley Electric Association pays for its electricity, meaning a dam would translate to lower electricity costs in the Interior.

The multibillion-dollar dam likely would rely on a mix of loans and public money. It would feed a half-dozen utilities between Fairbanks and Homer. Early cost estimates run to $4.5 billion, but Bryan Carey, a state energy engineer, said a design change could lower that figure.

It's generally believed a major dam is the only way to meet the Legislature's 2010 goal of shifting half of Alaska's electric power to renewables by 2025.

"Pretty much, we're believing the only way you can do that is a project on the Susitna River," Carey, of the the Alaska Energy Authority, told the Senate Resources Committee. He later told a reporter his agency expects to work with utilities soon on financing options.

Carey said the dam could operate for a century or longer. He said he's reviewing a new construction option - the use of compressed concrete. He said it could be simpler than the "embankment" option previously discussed.

Senators on the panel signaled they'll seek more information on the potential impacts on wildlife. The dam would create a lake 39 miles long and two miles wide. Carey said the proposed Watana dam, as it's called, would have little impact on fisheries or surrounding property use.

Another option - building at Chakachamna Lake near Cook Inlet - would carry broader environmental implications, he said.

Sens. Joe Thomas and Joe Paskvan, both D-Fairbanks, said they'll wait for more information on the concrete option, which Carey said would let engineers expand when or if it's needed.

"It's cheaper, it generates revenue earlier, and it has the potential to have no material impact" on the project's outline, said Paskvan, the committee's co-chairman.

The energy agency derived Wednesday's estimates using assumptions that construction costs would be split between state capital investment and a 30-year financing plan carrying a 6 percent borrowing rate.

Managers last year discussed borrowing for the entire thing, but Carey said Wednesday that investors will likely want to see a down payment from utilities or the state before they'll buy bonds.

The energy authority has said utilities and the state will eventually need to improve transmission lines between Fairbanks and Southcentral, and Carey suggested the dam will influence that need. But he said Watana would replace aging power plants elsewhere - suggesting the influence could be marginal.

The proposed project was planned in the 1980s but shelved when oil- and gas-fired electric generation remained cheap.

Both the state Senate and House have renewed focus on the idea.

"Twenty years from now, when they look back at this Legislature, I don't want (it) to be the one they say, 'If they'd only built Susitna,'" said Rep. Craig Johnson, R-Anchorage. "I've talked to legislators from the '80s, and that's their regret, that they didn't pull the trigger at the time."