The most recent release of WIlbur can do a flow analysis and threshold it to rivers for you. It would require a heightmap of some sort to start rather than an image, though. http://www.ridgenet.net/~jslayton/software.html if you'd like to try it.
Thanks for the input. I'll give a try.
rmfr
Perhaps imagination is only intelligence having fun. - Albert Einstein
A good friend will come down and bail you out of jail. A best friend will be in jail with you and say, "Dude, we screwed up."