According to the Indiana Department of Environmental Management, diesel fuel was dumped into the White River.
The fuel was released Wednesday night by the Indianapolis Power and Light Petersburg power generating plant.
IDEM says the issue was discovered Tuesday morning and stopped.
Residents may notice a petroleum sheen on the water and an odor.
The public is asked to avoid the area until both are no longer present.
The area of the White River affected runs from Petersburg to the Wabash River.