@Kelvin
I should point out that I’m not a professional geographer, but I’ll help where I can.
The very first thing you should do is decide on the scale: is this a continent (as would be suggested by the nomads you mentioned earlier, and the apparent multitude of environments), or a small island (which could be crossed in a single day)? The latter couldn’t really support five separate cultures (or even five separate nations in the same culture), while the former would take months to cross, even on horseback (and would make that bridge longer than any we have in the real world, which isn’t necessarily bad; I like the idea of a bridge it takes several days, if not weeks, to cross).
Either way, the lake/inland sea is very large compared to the land mass. Generally, one should assume that water “wants” to flow out to sea, and thus to keep the lake that large, you have to stop the water from leaving (or at least keep it slow). The river however, is very thick, with a very large delta, which implies that a lot of water is flowing through it. It’s possible that the lake is replenished by rainfall from the mountains, but I don’t think that would realistically be enough. I think that there are two ways of solving this while leaving the map basically the same.
The first would essentially make the lake an inlet of the sea (so water does flow out at low tide, and back in again at high tide). The former river could be thickened, and a few more “rivers” could be added between the lake and the sea. This would make the lake salty, and would create some very interesting tidal effects (whirlpools, etc) in the river. The second (and likely more appropriate) method would be to make the river much narrower, with a much smaller delta.
As I said before, you could just say that the gods made it that way (or use some other magical explanation), and it would be fine. I hope this is helpful; good luck with your story.
Edit: didn’t see your second reply – size of Europe sounds good for the various cultures (but you can’t ride across it in a day). If it’s just for approximate positions, then there’s no real problem beyond that.