forum.freeciv.org

When a river is next to a coastal city (which is not on a river tile), a Trireme can move to/from the city to/from the river tile, but the graphics as drawn don't reflect that. Ditto with a river tile next to an ocean tile where the river doesn't flow directly into the ocean. In the 1st attached screen shot you can see a Trireme which can move down to the left into the city, or up into the ocean, 2nd screenshot shows what it looks like without the unit.
This isn't a big problem, but it is more of a problem when a custom ruleset has canals. They display the same way as rivers, but I think they should show that they link up with neighbor tiles that units can travel to, since canals are constructed rather than pre-existing. Having all river tiles next to ocean tiles display as linked is reasonable - river deltas are a thing. Next to a city might be a bit weird.