What should you do when you are traveling upstream on a river at dusk and you see a buoy with the number four in a flashing red light on it?

When traveling upstream and encountering a buoy with a flashing red light and the number four, you should recognize that this indicates a hazard or a danger area, typically marking the edge of a channel or an obstruction. It's important to maintain a safe distance from the buoy and reduce your speed to navigate cautiously. Ensure to stay vigilant for any other navigational aids or hazards in the area as visibility decreases at dusk. If uncertain, consider turning back or seeking a safe place to anchor until visibility improves.