Two power plants previously owned and operated by a company formerly known as Mirant Delta, LLC were located in the Delta towns of Antioch and Pittsburg.  Now part of NRG Energy, the plants generated approximately 600 MW and 1,300 MW of natural gas power, respectively. Both plants utilized controversial once-through cooling systems, which divert water into the plants then return the water back into the Delta.

When in operation, the plants drew in aquatic organisms including at-risk fishes, generally resulting in mortality.

Use of the plants began declining steadily starting in 2002, but at that time their impacts on at-risk species were not well understood.  The plants’ operations fell under heavy review and scrutiny and were studied for their potential impacts on delta smelt and other species protected under the federal Endangered Species Act and California Endangered Species Act.

In 2010, the Coalition, Kern County Water Agency, and the United States Army Corps of Engineers entered into a settlement. The settlement provided for increased monitoring of the aquatic impacts of power plant operations and a specified timeframe to complete consultation with the U.S Fish and Wildlife Service (FWS) and the National Marine Fisheries Service (NMFS) regarding the impacts of power plant operations on endangered and threatened fish, including the delta smelt. Ultimately, due to the efforts of the Coalition and others, once-through cooling systems became known as an environmentally harmful practice.

In a positive step, NRG Energy shut down the plants in 2013 and replaced them with  plants that employ practices less harmful to the Delta ecosystem.