First, let's hope that the person trapped is still fighting, if so, get the line as low as you can, and I am betting they will grab it, pretty hard too. Be sure you have several people on each end, and/or a no knot or other way to secure the line for their weight the the water pressure.  

Once they are heads up/stabilized, you should use a different line to try to untrap their foot/leg/etc. As someone already said, that will be more difficult. Rocks in a bag tied to the center of your second line sounds like a reasonable idea, use whatever you have. You may have to do a cinch or other trick to pull in various directions, but try the simplest things first because the trapped person will be getting tired. There are several reasons in this paragraph to carry not one small rope, but several ropes, with one being at least 70 feet.

Lots of other things to do here too, like upstream safety.

Try to find a relatively safe place you can try this stuff and see for yourself what works.

Might be time to get in on a SWR class before it gets cold too.


edgar