Possible Clues:

• Like most bicycle tires
• Floated downstream, in a way
• Floated down a river, say
• Went down the Chattahoochee, parhaps
• Went leisurely downriver, perhaps
• Rode down a river, in a way
• Took the London subway
• Floated down the river, in a way
• On the way up first appearance enclosed in pipe
• Went downriver, one way