You could use a hybrid solution: have the randomness be inclined towards grass as the terrain approaches the river, while on the other side, allow more dirt (with a few pockets of grass thrown in). Mix the different terrain types, have a few rough triangles/trapezoids of dirt cross the grassy area and touch the river, but don't make it entirely random - keep it at least somewhat reasonable, with unclear regions that intersect each other. Other than that, try not to use such long and linear borders, especially not if you have them meet at straight angles... It just looks like you're using the larger brushes, and makes it feel a bit like a chessboard. | 
