The act of performing a bowel movement in any body of water, under complete anal submersion. Can also be used as a noun to describe product of said bowel movement. Best done downstream.
Timmy rounded the river bend, preceded by a floating turd. "Dude, where have you been?" I asked him. "There were no bathrooms. Had to pull an aquadump."
Used in the Wilco song I Am Trying To Break Your Heart. Jeff Tweedy is saying that he is a heavy drinker. A figure of speech meaning he drinks as much as an aquarium could hold.