The accident was recorded on-camera as an activist was filming an interview, ABC News.
Erin Schrode who describes herself as an activist journalist said Thursday she was fired at the back at the Dakota Access Pipeline protest.
She reported on Facebook: "I was standing innocently onshore, not making any aggressive gestures, never exchanging a single word with the police who fired at my lower back from their boat."
Schrode complained about what she called "the indiscriminate use of excessive force," in the post.
"Authorities used less-than-lethal ammunition to control the situation," the Morton County Sheriff's Department said in a statement. The Sheriff accused one protester of throwing bottles at police and another of refusing to disperse and charging at a police line.
"No lethal shots were fired from law enforcement," the statement emphasizes.
Earlier, oil pipeline
protest in the US involved clash with riot officers.