If you have ever tried to ferry a line across a fast moving and wide river, you know this can be a real challenge.  First off, get a person or two across to form the receiving party.  If there is a narrow section reasonably close, you can often toss in the normal manner.  Once received, each side simply walks the line upstream to where it is needed.  Sometimes you can cheat and wade into the water to an eddy or rock to reduce the throwing distance.  Since everyone is supposed to carry a throw rope, have someone toss you another and tie together with an inline figure 8 knot.  Sometimes you need to ferry the line via a boat.  If available, use a raft or canoe so one person can concentrate on keeping the line out of water.  Start high on the shore as well.  Thin lines provide far less drag than thick ones.  This is a great use for multiple 1/4" lines ties together.  Once across, you can simply tie the big rope on the end and quickly pull across.  When ferrying a rope, be extremely generous with rope length.  If the crossing is 100', tie together 2 or 3 long lines as the boat will most likely drift downstream.  Since the line typically drags a boat downstream, plan ahead and start high in the rapid/current and ferry quickly on an angle downstream.  People on the other side should be ready to grab the boat or rope ASAP.