In an effort to get a jump on pending environmental rules, Colorado regulators in late December approved plans to retire six coal-fired units at two utilities, switch two units from coal to natural gas and build two natural gas-fired powerplants. Photo: Xcel Energy Three units at Xcel Energy's coal-fired Cherokee power plant in Denver Driven by a state law, Minneapolis, Minn.-based Xcel Energy plans to spend about $1 billion by 2017 retiring 591 megawatts, switching 700 MW from coal to natural gas and building a 570-MW gas-fired plant. The utility will also add pollution control equipment at two powerplants. Xcel
Arch dams, classically curved monoliths situated in narrow valleys, are not as common as other types of dams, such as embankment dams and gravity dams. ENR takes a look at some of the biggest arch dams in the world, both completed and under construction.