But the dam in recent years hasn't produced as much power as it might: Its massive turbines act as deadly blender blades to young salmon, and engineers often have had to let the river flow over the spillway to halt the slaughter, wasting the water's energy potential. The ability of the nation's aging hydroelectric dams to produce energy free of the curse of greenhouse gas emissions and Middle Eastern politics has suddenly made them financially attractive -- thanks to the new economics of climate change. Armed with the possibility of powerful new cap-and-trade financial bonuses, the National Hydropower Association has set a goal of doubling the nation's hydropower capacity by 2025. Expanding hydropower is fraught with controversy, much of it stemming from the industry's history of turning wild rivers into industrialized reservoirs struggling to support their remaining fish. The emerging boom in hydroelectric power pits two competing ecological perils against each other: widespread fish extinctions and a warming planet. The issue has been particularly contentious in the Pacific Northwest, where some are calling for breaching dams on the Snake River in an effort to bring back the declining salmon and steelhead. 'Hydropower does have pretty significant and serious impacts on rivers. We know that. The industry knows that,' said John Seebach, director of the Hydropower Reform Initiative launched by the conservation group American Rivers. 'It also provides some pretty significant benefits in terms of power production. So it's a tricky balance to get those benefits while trying to minimize those impacts.' Across the country, there are about 82,600 dams, but only about 3 percent of them are used to generate electricity. Hydropower produces about 6 percent of the nation's electricity, and nearly 75 percent of all renewable electric power. The increasing mandates for power utilities to expand their portfolios of renewable energy are prompting dam operators to take a second look at thousands of dams now used for flood control, irrigation, navigation, recreation and industrial water supply that might also be used to generate electricity without further harm to fish. 'Most of the bang for the buck is at existing dams and reservoirs without hydropower facilities, and hydropower facilities that need to be upgraded for additional capacity,' said Norman Bishop, vice president of MWH Americas Inc., which designed the dam improvements in Chelan County, Wash., home to the Rocky Reach facility. The U.S. Department of Energy estimated that there are up to 30,000 megawatts of potential energy at 5,677 undeveloped sites across the nation, more than half of which already have dams. Newly added to the equation is the emerging market for so-called carbon credits. The credits are part of a strategy to place 'caps' on damaging greenhouse gas emissions while allowing companies that can't meet the restriction to buy credits from ones that achieve significant savings. The cap would be gradually lowered to reduce overall emission levels. Hydroelectric power is a prime candidate to sell credits because it is largely emission free. The credits typically would be granted for only new or additional power. The market for the credits is tiny now, but legislation is moving forward that would create caps and a national market that could ultimately reach $120 billion a year. Even without a national cap-and-trade law, markets such as the Chicago Climate Exchange now allow companies to voluntarily limit their carbon emissions and lower their carbon footprint by purchasing credits, traded on the market like stock. This added incentive has made building or upgrading hydroelectric facilities a more alluring prospect. The small rural Chelan County Public Utility District last year became the first hydropower facility in the U.S. to begin trading carbon credits on the Chicago Climate Exchange. The money the district has made from selling credits -- about $1.6 million so far -- is going back to Chelan County and its customers for new investments in carbon-free electricity. The district has invested heavily in making sure its new electricity results have no net harm to salmon -- a key requirement for trading on the Chicago exchange. But the possibility of more hydroelectric construction around the world has set off alarm bells among some groups of environmentalists. (Begin optional trim) 'Rivers in the U.S. have been seriously impacted by dam construction,' the conservation group International Rivers said in urging California authorities to disqualify hydropower projects producing more than 10 megawatts of power from receiving carbon credits. 'Fortunately, some of this damage is now starting to be reversed by dam removals,' the group said. 'California climate action should not act as an incentive to increase damage to rivers and prevent efforts to restore them.' California receives about 9.6 percent of its power from large hydro generators. The state has said it will consider as renewable energy only those hydro projects smaller than 30 megawatts that do not require the diversion of any new water. (End optional trim) Climate-change activists particularly balk at the idea of offering carbon credits in the U.S. for large hydropower projects in developing countries, such as Chile, Peru, Uganda and elsewhere, where environmental protections may be lax and the overall contribution to global welfare dubious. But here at Rocky Reach Dam, engineers say they believe there is a way to reduce emissions, increase power output and save fish at the same time -- although at a cost. The Chelan County utility district spent $292 million overhauling Rocky Reach's 11 aging generators and installing new, more efficient turbines and an expensive mile-long safe-passage tunnel for up to 3.5 million young salmon and steelhead that navigate the dam each year. With the juvenile-fish passage facilities -- along with commitments to improve habitat and expand hatchery production for salmon -- the district could meet its targets for healthy fish and allow much less water to spill over the dam. Five years ago Rocky Reach had to spill up to a quarter of its water over a 31-day period during the height of the spring salmon juvenile migration, but last spring it received permission to spill no water at all. Yet more than 90 percent of the young salmon and 94 percent of the steelhead are surviving their trip past Rocky Reach Dam, according to district records. The result is that the dam has been able to produce an additional 1.75 million more megawatt-hours of electricity over a recent three-year period, the equivalent of 702,204 metric tons of carbon if the electricity were generated at a natural-gas-fired power plant. 'What we have been able to do is provide more power with the same amount of water,' said Tracy Yount, the Chelan County utility district's external affairs director. 'We're saying, let's skip the new facilities, skip the regulatory issues associated with new dams and go to our existing facilities and get more value from them.' 