What does "boats can go no higher up this river" mean?

What does "boats can go no higher up this river" mean in this paragraph?

The port of Manham is located in beautiful and peaceful countryside, on a bend in the great River Avon, and developed here because it's the highest navigable point of the Avon - boats can go no higher up this river - and proved a handy place to load and unload cargo to and from the sea, which is over 23 miles away.

boats can go no higher up this river

The statement means that boats can travel no further upstream (i.e. away from the mouth of the river). Since rivers naturally run downhill, this literally means that boats can travel no higher (uphill) using that river.