When our river rafting guide dipped his water bottle into the Río  and took a big long swig, I shuddered out of habit. You just don’t drink untreated river water in most parts of the world.  That’s backcountry lesson number one, right? But Aysén isn’t like most parts of the world. In this pristine part of Patagonia, the blue-green water streams directly from the Northern Ice Field, unpolluted by livestock or industry or any other waste-producing entity. And so, pushing aside the fears of microscopic critters invading my belly, I leaned toward the water, scooped up a handful, and slurped.

Few places on our planet remain as untouched as Aysén, where four immaculate rivers flow uninterrupted from source to sea. You couldn’t cross the region by car until 2000, when the gravel “highway” started by Pinochet in the 1970s finally made its way from Puerto Montt in the Lakes Region 770 miles south to Villa O’Higgins. The roadside scenery is still dominated by bushy lenga trees, calafate flowers, and an intricate network of rivers, lakes and streams, set against a backdrop of gleaming glacial peaks. Subsistence farming is the norm, and the population density is less than three per square mile.

Much will change if Spanish power company Endesa carries out its plan to build two dams along the Baker and two on the Río Pascua, in southern Aysén. Together the dams will produce 2,430 megawatts that will be connected to the national grid via more than 1,800 miles of new transmission lines, and shipped to power-thirsty central Chile. The project will provide some short-term construction jobs and a temporary boost to the local economy. It will also inundate up to 36 square miles of land, permanently altering the ecology and turning the upper Baker stagnant.

Regional and international environmentalists are working to halt the project, but the fact that Chile’s water rights are private limits their ability to intervene. Construction of the first dam is scheduled to begin in 2008. It will harness the hydropower of the Río Baker as it slams into the Río Chacabuco in a narrow, rocky canyon just downstream from the quiet cove where we pulled our river raft onto shore, rested on the rocky beach, then waded back into the icy water for one last drink.