River Length #2

You are in a river of unknown length that flows with an unknown constant velocity. If you stay stationary in the water (just drift), it takes you 6 seconds to be carried to the other end (the downstream end). If you swim against the flow at 3 meters per second relative to the water, it takes you 8 seconds to reach that same end (you are still carried downstream, only more slowly). What is the length of the river in meters?