Here's what happens:
* He's obsessed with chocolate. Augustus is a gluttonous boy who is constantly eating chocolate.
* He ignores Wonka's warnings. Wonka explicitly tells the children not to touch the chocolate river, but Augustus doesn't listen.
* He falls in and gets stuck. He leans too far over the river, loses his balance, and falls in. The chocolate is thick and he gets stuck.
* He's rescued by the Oompa Loompas. The Oompa Loompas use a giant pipe to suck Augustus out of the river and carry him away.
* He's still obsessed with chocolate. Despite his near-death experience, Augustus still wants more chocolate.
This incident is a classic example of the consequences of greed and ignoring warnings. It's also a moment that highlights the whimsical and slightly disturbing nature of the story.