Useful Businesses Post-collapse: Ferrying travelers
Some enterprising people might utilize their proximity to a river and high traffic to create a service industry. Lots of people will come along from trading goods and migrating. Bridges might be out or commandeered. In good weather people will ford across the shallow sections, but it's riskier as their wagons might get hung up and hence abandoned. Or service animals can injure themselves as they pull along the river stones and lose their footing.
Often these were modified flatboats since they could haul so much on their flat surfaces. Then perhaps there would be a pulley system set up to pull the craft along in a controlled manner versus using poles when the craft was so heavy. This meant the ferryman had to maintain the equipment carefully from fraying and lubricating the pulleys and making sure all of it was secure too.
Of course people mostly traveled during the day, and if you came along at inopportune times, the travelers were stuck until the ferryman came back out to resume service. Naturally some taverns set up on either side to assist weary travelers wanted to sleep in a real bed.
Almost always in history, cities began as tiny tribal villages along river banks. Tribes need water for themselves and for crops. Game animals need that water too. It meant fishing, sturgeon egg harvesting, mussel hunting (and buttons and calcium sources for treating corn flour), beaver pelts, trade, etc. I have no doubt that post-collapse, it will all happen again as it has over and over in history, and so some smart folks may use the river to their advantage.
[link to en.wikipedia.org
[link to digital.library.okstate.edu