[unkated]

I imagine that towns would mark navigable channels on the approach to their docks, and keep those fairly up to date to encourage traffic.

I suspect that with a little experience, you would get an idea of which areas of a river may vary and which will remain fairly stable based on size and shape of banks, type and depth of bottom, straight or curved etc.

Boats would have to navigate the old fashioned way - by checking as they go if they aren't sure.

Which brings me to an NPC for a Vistula campaign:

Marco Czerty
Nom de guerre (it's on his paperwork; he paid good money for that) of an ex-Polish merchant marine sailor now working on a small river steamer like the Wisla Krowola. May have spent time in military and have detached voluntarily (hence working under his current name). Some skill in Boats, Navigation, Small arms, Mechanic.

Tells interesting and somewhat amusing stories of current life along the river, twisted versions of Polish folktales, wry looks at the (ex-)Communist gov't and the Soviets. (see Collected Stories of the Twillight, published Lublin 2025).

"Marco Czerty" translates to "Mark Four" - 4 meters being close to the two fathom safety margin for shallow 19th C river steamers that gave Samuel Clemens his pen name - Mark Twain.