While it is true that salmon eventually stop "actively" feeding once in the river, the reason is not because of hitting brackish water. They stop feeding when they begin to utilize thier internal organs as energy reserves to fullfill their destiny. This is more a function of water conditions and the condition of the fish. The darker the fish the less interested in feeding it becomes. I have caught hundreds of spring salmon (cromers) on the Trinity River, a hundred miles upstream, that were still actively and voraciously feeding without a doubt. Stomachs just stuffed full of discarded roe when you clean them.