We know that River is the stand-alone biome, and the rain is allowed in it. Sometimes river streams along/between the Desrert and Savanna biomes. Neither Desert nor Savanna biome allows rainfall. But if the river divides these biomes, we can see the rain just above the river, and the dry weather in the desert / savanna nearby. It's like the rainy path between two arid landscapes I think that such a scene is absurd.

The game engine must check the rainfall regime in the adjoining chunks, which appertain to no-river biome. It should be done for each chunk with river biome.
1. If the river chunk doesn't border any rainy biomes, it mustn't raining in this chuck at all.
2. If the river chunk borders at least one another chunk with rainy weather, it can be raining it this chunk.
I suppose the game logics must be changed to solve the problem by this way... That's problematic too. So the developers can solve it by other way - adding a new river biome (for example, "dry river" with no rainfall). It may be done by analogy with "ice river" biome.

And, finally, fix the "nonperennial river" phenomenon Almost all the rivers are interruptive. It confuses when I can't sail down the rivers with boat because of it.