Duke says no water was flowing through the dam.
Steve McEvoy of the state Division of Energy, Mineral and Land Resources says inspectors are trying to determine the cause of the crack, and whether the opening is a threat to the integrity of the structure.
But he says the dam doesn't appear to be in imminent danger of failing.
The structure is at the Cape Fear River plant, where North Carolina regulators said Thursday that Duke illegally pumped 61 million gallons of contaminated water from a coal ash pit.
Regulators say the dumping violated Duke's wastewater permit at its Cape Fear plant.