The wastewater treatment plant in Medford, Ore., removes organic solids, oils and other pollutants from sewer and storm runoff before dumping the water into the Rogue River. Even though the process cleans the water, it's still polluted with heat. Warm waters hold less oxygen and can provide a dangerous advantage to invasive species. The state allows each heat-pollution source in a watershed to contribute a maximum number of kilocalories per day so river temperatures won't rise above 13 degrees Celsius, the ideal temperature for salmon to spawn. Required by law to cool its wastewater, Medford considered traditional solutions, but refrigeration units or cooling towers are expensive and rely on power generated by river-blocking dams or coal. Instead, the city opted for an innovative new approach -- restoring trees and shrubs on riverbanks left bare by logging and agriculture. The resulting shade keeps the water cool naturally, at a cost