A St John spokesman said St John received a call at 11.16pm from another person saying that someone was "holding on to a log and floating down the river".
"According to the person we spoke to, they had received texts and calls from the person," the spokesman said.
"We sent two Jeeps but the person was not located, so it has been handed over to police."
He added: "There might have been doubts cast on it all."