Complex sentence: Plutonium for nuclear weapons was produced for decades at the Hanford Site, which is now the most contaminated nuclear site in the U.S. All of these developments have had a tremendous impact on river environments, mainly through industrial pollution and barriers to fish migration.
Split into simpler sentences: Plutonium for nuclear weapons was produced for decades at the Hanford Site, which is now the most contaminated nuclear site in the US. All these developments have had a tremendous impact on river environments, mainly through industrial pollution and barriers to fish migration.