A line that crosses the river has many uses:

• A floating tag line (line with a float like a life jacket) provides a safe way to assist someone stuck in a hydraulic like a low head dam.
• A stabilization line can provide support for a trapped boater to help them keep their head out of the water.
• A snag line (weighted in the middle) can be used to safely free someone from a foot entrapment.
• Equipment and people can quickly traverse the creek via a line crossing. Here is a video of a zip line crossing: Zip Line Crossing.
• A variation of this technique is used to safely lower a boat as a platform or a rescuer in a rescue PFD.

This is a great skill to master.