They are set in the water type window. Open the window (Look at... RiverWaterFlowSE for example) and in the bottom left of that window there are 3 vector force things. The z is unused I think, and the X and Y determine the direction and strength or the flow.
A tip; 1 is a fairly strong current. I start at 10 and it slammed me around at like Mach 5. So try something like .1 or .2 for a gentle slow stream, .5 for a bit faster, and 1-2 for a quicker river.