A classic case of perspective mix-up
As soon as I saw the video for this "river monster" in Saskatoon, I knew exactly what it was. This is a classic case of a perspective mix-up, which has caused countless false river monster sightings over the years.
If something long and flexible gets snagged in a river, it trails out in the current, and the movement of the water makes it look like something swimming. This happens most often with lengths of rope (as it did here). The rope gets snagged on some branches, and the way that it waves in the current makes it look like it's alive. If the rope in turn snags blobs of trash or vegetation, that just makes it look more monster-y. The same thing ca happen with tarps, plastic bags, and other trash.
The key is to watch the "head" of the monster. If the head does not move in the current relative to the banks of the river (either left or right, or forward or back) then it's probably just some snagged trash.