My daughter saw its head. I saw its body and tail only. Between the two of us, we agreed it was the right shape and length (over one metre, with a very long tail) that it could not have been a beaver. It was hanging out under a bridge in fast moving water in the early evening, leaving a trail of bubbles as it dove and surfaced on either side of the bridge three times before it disappeared. This is the only picture I managed to snap.