From WoWWiki
The Poison Falls are located inside Maraudon, and is the meeting point of the physical corruption stemming from the Foulspore Cavern and the demonical taint stemming from the Wicked Grotto. The two forms of pollution mingle here, and result in a cascade of foul, greenish water that is on the border of becoming slime. The cursed Celebras stands guard over the falls where they border on the pure part of the dungeon. Beyond lie the gloriously clean Earth Song Falls in sharp contrast to the vile water found in the Poison Falls.
