I was really excited to read this post.. I saw something similar in Valley creek within the last year. Seems like a similar spot too (between Knox bridge and iron bridge).
It was the morning after significant rainfall and the water was pretty turbid. I looked upstream between casts and saw what I thought was a good-sized log floating down the center current. It moved and I made eye contact and thought “whoa that’s an otter!” It dove and swam downstream, passing maybe 10ft in front of me, barely submerged. As I tried to get a better look at it, the two things that stuck out were
1) it had a long thin tail, not the paddle tail of a beaver
2) it was much bigger (3 ft?) than the mink I see there, but had a similar sleek build (not as stocky as beaver/muskrat)
After the fact, I started to second guess myself, since there are many mink and beaver in that creek, but no other mentions of otters that I could find.