River Currents

I can easily make a river. I just don't know how to make a current.

All I want is a shallow (not deep enough to count as underwater) river, with a current that pulls you along to the end. Can someone explain how to do that?

Once I put this river in, my new stage will be complete, so please help me out with this one issue.

http://wiki.srb2.org/wiki/Linedef_Type_544

The direction of the linedef is the direction that the current is going to move. If I can remember correctly, that direction is the direction in which you draw it. Starting from the point you started at, and going to the point you end it at. The length will determine the speed.