At a time when large dams are being taken down, not put up, the state of Alaska is proposing to construct one of the tallest and most expensive hydroelectric dams ever built in North America.

The Alaska Energy Authority is planning to build a 735-foot, $5.2 billion structure on the Susitna River in a largely empty south-central part of the state, which is watered by runoff from the arc of the Alaska Range. The dam, designed to generate up to 600 megawatts of electricity, would create a new power supply for more than two-thirds of the state’s population.

But in Alaska, where natural energy resources and wildlife are both foundations of the economy, the proposed dam presents twin conundrums.

One is economic: which is better, creating a reliable source of hydroelectricity and weaning some of the state off natural gas, or building a spur off a proposed pipeline to bring gas from the North Slope to the populated region from Fairbanks to the Kenai Peninsula? Or both? The other is environmental: what serves the environment best, replacing natural gas-fired electricity with hydroelectricity, which is free of greenhouse gas emissions, or keeping the Susitna watershed untrammeled and avoiding the risks involved in changing the dynamics of a major salmon stream?