The ash basin dam along the river is secure and downstream municipal water supplies remain unaffected.
Crews are working on a plan to stabilize the area and to determine the volume of water and ash that reached the river.
The coal plant retired in 2012, so the basin was already at a lower level. The primary basin is about 27 surface acres.
Duke Energy has notified a water quality monitoring team. The N.C. Department of Environment and Natural Resources is also performing a site inspection.
The company has been looking for the most appropriate way to close the ash basins retired plant sites.