As a general rule, it is best to get a boat or person out to the entrapped victim. Chances are they are injured and hanging on for dear of life. The constant force of water pounding on them will wear them down eventually - time is critical. While safely getting someone to the victim, others can work on getting a rope across the river to work as a stabilization line. Once the victim is stabilized and no longer in danger of drowning, work on freeing them. You may need to manually free their foot which means putting yourself in danger as well. Make certain you have safety in place such as a tag line on a rescue vest. Safety will need to be set both downstream and upstream. Take very careful scraping steps and use a paddle or rope as a third leg. Once the victim is freed, they will certainly need help getting to shore as they are most like injured.