Do rivers have distance markers?

No because on a river or waterway the pilot/captain has to use charts and RADAR. There are markers or buoys but they have a number that is used to find where you are on the chart.

None that I’ve seen. River boat captains travel the same river often enough to know how far everything is. Not too many newbs on the river to get lost…And how could anyone get lost on a river???

Yes. Many do. On the Edisto River, they are duck boxes, with a number representing miles to the ocean.
In inner coastal waterways, they are green, and redish orange. But aren’t mile markers…

Samuel Langhorne Clemens was famous for one type of distance marker.