That set off a chain of events that extends far beyond the river banks.
On the river, what you see depends on where you look.
The surface water, which is constantly flushed downstream, looks good and experts say often tests within EPA standards. But get down into the sediment and it appears to be a different story.
Brian Williams, a long-time river guide with the Dan River Basin Association, said the water doesn't look polluted.
"On the surface water, you're not going to see a lot of the contamination anymore because the sediments go down to the bottom," Williams said.
That's what Williams is most concerned about -- what's on the bottom. He dug through sediment churned up in recent rains.
"You can see this greyish material here -- that's coal ash, yeah," Williams said. "And then you can see there's nothing here. It's all mixed in and you see, that's the insidious thing about this."
Williams is keeping a close eye on mayflies near the river. Mayflies act as barometers of the river's health and predictors of what's to come.
"You can see the little gills going right there - that's the way they take in oxygen," Williams explained as he pointed at the fish. "So you can imagine with a little bit of sediment or a little bit of coal ash, these little guys can't even breathe. And then if one small minnow eats 50 of these and one bass eats 20 minnows, then you, know, you've magnified the effects of the toxins in there."
Duke Energy was responsible for the spill and has been vacuuming what ash it can off the bottom of the river. But that only works when there's a lot of ash in one place, and so far they've only removed a fraction of what spilled.
The upshot is they've been testing the water, and a spokesperson said Thursday that water quality has returned to precondition levels on the Dan River.
But Williams has his doubts.
"How do they know that?" he wondered. "That's the big question. I think it's not a responsible thing to say that the river is back to normal and it's clean. We're not saying you're seeing an acute effect right now, fish dying or anything like that, but we are saying it is affecting the environment. The material is still in the river. The material is still moving its way down the river and it's still causing the same problems."
Williams thinks there's coal ash two miles upstream of the plant. How can that be?
The ash there is now dry and Williams thinks it's blowing into the river. He's waiting on sampling to see whether there is coal ash there. If it is, that will be one more thing Duke Energy will have to answer for and - potentially - clean up.