Currently at this time, there are no natural rivers present on the map. The waterway that connects the Inner Sea with the ocean was constructed by the humans that live on the western island. They originally used the waterway to access the sea and trade with the kingdoms further inland.

They eventually lost control of the waterway to cabal of wizards that altered it with magic and raised the island that is now present there. They then established their own domain here.

And yes, I know. It's green. But, please forgive me, I like it this way. I may decide to lighten up the green later, but it depends on how the map develops. (I like cartoony colors in my maps. )