Yes, happens every year. The life cycle of the salmon: hatch, swim out to sea, eat, grow, swim back up the river you came out of, have sex, and die.
"...sort of like feeding the bears,..."
I think that is the idea. Lots of salmon equals happy bears. Just imagine, all those bears waiting around the river for the salmon to show up.
the predators like whales that will eventually learn to come there to feed on the baby salmon will become trained to do so and will increase in number.
Which will result in a mass die off of the whales if the practice was suddenly stopped.