But, according to the Buffalo County Sheriff's Office, it was a false alarm.
Archway Monument staff called authorities after a man fishing nearby alerted them to the possible cougar shortly after noon.
"This guy came in and he was really kind of disturbed, seemed frightened. He said that a mountain lion had run him off the lake and he was coming in to tell us about it, warn about it,” said Jack Garrison, an Archway reenactor.
Game and Parks was called and determined that the animal had likely been a large dog.
The Archway had already taken precautions.
"Immediately one of our staff went out and stood on the bridge so that nobody go over there, turn people away,” said Garrison. “Then the staff brought in barricades and put them up."
It didn't stop people from visiting Saturday.
Garrison said the Archway has had a good crowd so far this weekend.