There are several ways to throw a line across a river. Underhand typically works best for long heavy lines. Overhand like a baseball pitch does well with smaller 1/4" lines. Sidearm may be necessary to avoid overhanging branches. These are your typical direct throwing techniques. In some cases you may need to get someone part of the way in the rapid, perhaps on a midstream boulder. If he brings two lines, he can toss to both shores and tie in the middle. Alternatively, someone can toss to this person. He can then take up the slack and rethrow to the other side. This buddy throw technique is great for bypassing obstacles. Rescue professionals use a really long thin line called a messenger line. Parachute cord is one example. You can tie a small weight on the end and toss a very long distance. Once across, tie your primary line on the end and quickly pull across. We can improvise to some extent with personal throw lines (the 1/4" ones) by tying several together.