I am wondering what kind of animal I seen in the Stanislaus River...?

One night a few years ago, I saw hundreds of animals in the river. I thought they were beavers, but I seen pictures of an otter, and it looks more like an otter. They swam with their heads above the water looking around, their coats were shiny, they had whiskers, and they were so cute! I seen them in the Stanislaus River in Modesto, CA, and I want to know if anyone else has seen this before, and what they were? Was this a one time herd... or whatever or a yearly thing... I don't know exactly when it happened, but I know it was a cold month.... I wish someone had a video of them...