Some of the rivers in Middle-earth form the border between two differnt biomes. But only optically.
If you look close enough, you'll notice, that the border meanders across the river, so that small patches of the biome leak over to the wrong side of the river. This irritates me a lot, as this means mobs, vegetation, even buildings can also be found on the "wrong" side of the river.
Since Public Beta 29, rivers are no longer a biome, but only a variant. This allows narrower rivers, which looks really fine, but also leads to them no longer separating two adjacent biomes.
Here is another example, where the Shire leaks across the Brandywine into what should be Eriador.
As Mevans explained, each dot on the ME map defines 128 blocks (8 chunks) of the world. Which caused rivers to be at least this wide before Public Beta 29.
I assume, that the borders of biomes are allowed to meander thus far, if two adjacent map pixels do not belong to the same biome. That means, if a river is only 2 chunks wide, the patches on the "wrong" side can reach in up to 6 chunks.
Now my question: Is it possible to find an easy tweak for the terrain generation to prevent the effect described here?