It's a water quality issue. They don't want hundreds of thousands of dead sockeye above the Seattle Water intake. Other species are few in number; they aren't likely to cause problems. Most of the steelhead will swim back out, anyway. Recently, the sockeye fry from the Landsburg hatchery project were released. Scientists diving in the river, near Landsburg, reported there were large cutthroat behind every boulder gobbling up fry.