Puzzle No. 001

The Harvard bridge and the Longfellow bridge are 1 mile apart. The MIT crew team starts rowing upstream at the Longfellow. As the crew passes under the Harvard, the coxswain's hat falls into the river. 10 minutes later, the coxswain notices and turns the boat around instantaneously and has the crew go back to get it, rowing at the same constant rate. By the time the team reaches the hat, they are back at the longfellow.

How fast is the river flowing?

Source: rec.puzzles.FAQ