On Oct. 18, he decided to walk his camera down to the University Avenue bridge that crosses the river to snap a few pictures of ice forming on the water's surface.
But instead of finding just snow and ice, McCaa saw a "strange thing" moving back and forth in the current of the murky river.
"It never fully revealed itself," he said in a phone interview Tuesday.
McCaa took a video and posted it to the "Bureau of Land Management – Alaska" Facebook page Tuesday afternoon. By evening it had been watched about 20,900 times. Speculation has run rampant on what the creepy object — or creature — swaying in the water might be.
"I initially thought, and several people thought, it could be some rope that snagged on the bottom of the river with chunks of ice," McCaa said. "Other people have mentioned everything from sea monster to giant sturgeon."