[bmickler]

The start and finish of the race are located at fixed points. If you're racing on tidal rivers near the coast, where most of our regattas were, the current varies widely throughout the day. Say it's a 5k head-race starting upstream and coming down towards the sea. At high and low tide you're essentially racing with no (or very little) current - much like a lake. When the tide is coming in, you'd be racing against it and a 5k race can take 20 or more minutes. If you're lucky and your race heat starts when the tide is going out, that same race will take 15 or less minutes.