It is not all that strange to see any salmon in small streams, good percentages of all salmon wander, not returning to the same place that they hatched at.

And chums prefer to spawn in small tributaries or side channels to the main river.

The wierd thing about your fish is that it tried to get up that revetment for such a small flow. But I have never been one to completely understand fish.
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Give a man a fish, and you'll feed him for a day; give him a religion, and he'll starve to death while praying for a fish.