I'm thinking he dumped it ABOVE Whiskey Bay now. Accounts for the bike floating to the bank. He had to get up there because he had to get it into the Pilot Channel. Look at the map I just linked. If he didn't, it may have made its way down to Butte La Rose if it someone floated back up, which he didn't want. The water gets shallower and shallower.... get it past the bend into the Pilot Channael and he's home free - fast current and no population downstream!
He screwed up not deflating the other tire, or there would have been no trace.
JMO - MOO - but I think the theory holds water.
Check out the map again!
(Whiskey Bay at top right)