Complex sentence: A continuing controversy about these high dams involves Chinook salmon and steelhead blocked from roughly half of their historic habitat and spawning grounds on the Willamette's major tributaries, thus "brought to the brink of extinction".
Split into simpler sentences: A continuing controversy about these high dams involves Chinook salmon and steelhead blocked from roughly half of their historic habitat and spawning grounds on the Willamette's major tributaries. Unable to survive and reproduce as they once did, they have been "brought to the brink of extinction".