Remember the acronym RETHROG.  This article covers the forst two parts - Reach and Throw.Reach is pretty easy to understand, someone fell off a bank close by.  You can simply reach down and grab them.  For a bit more reach, you can toss them webbing you might be carrying with you.  Throwing is much more common.  A key piece of gear every intermediate whitewater boater should have is a throw rope.  Throw ropes vary in length between 50' and 70'.  It takes some practice to become skilled in tossing a throw rope but in an emergency, your practice will pay off quite handsomely.  You typically aim for the center of their chest - don't make them swim for a rope as that can be quite dangerous.  Once they grab the line, be prepared with a strong belay position to handle the shock load.  Whenever working with ropes, set safety above and below the rapid.  The upstream person is responsible for warning other boaters about the danger down below.  You should also have some sort of cutting tool like a knife or trauma shears to protect against entanglement.  Here is a short video with an example: Throw Rope Toss.