Bilbo discovered trapdoors hidden behind large barrels while exploring. The elves emptied the barrels and sent them through the trapdoors. The empty barrels were collected, tied together, and floated back to Lake-town on the river.

1. Barrels out of Bond A Summary by Graham Rounds

9. “ Hiding behind one of the largest barrels Bilbo discovered the trapdoors and their use… When the barrels were empty the elves cast them through the trapdoors… There they were collected and tied together and floated back to Lake-town.”