Brain Teaser: Math Problem?

A man is rowing upstream in a rowboat with a bottle of soda sitting on the stern. As he rows under a bridge, the boat bumps into a rock and the bottle of soda falls into the river, unbeknownst to the rower. Twenty minutes later while still rowing upstream the man discovers that his soda is missing, so he immediately turns around and rows downstream to find his bottle. One mile past the bridge he finds the bottle, What was the rate of current?