When a salmon comes into a river system they are full of energy.They’ve come here to party like its 2018! They have this very strong primordial desire to mate! Once they do, their energy levels begin to drop fast.(kind of like human males) Basically they begin to die & they know it.
When they get weaker they cannot swim out in the middle of the fast moving river.They come into the shallows to get out of that strong current. It would be similar to you running a marathon & there is a strong head wind against you! Getting out of that wind is all your thinking of!
Once they move into the shallows they become easy prey for the bears! The bears patrol the banks of the river each day.One might go upstream in the morning & maybe come downstream in the afternoon.
They are looking for salmon like our friend here.If a bear comes along,it’ll invite Mr.Salmon to dinner.They then will go for a short walk into the forest.