The River is a small expansion consisting of twelve tiles. It is included in the base game published by Rio Grande Games. These twelve tiles are designed to be played before the other tiles, and start with a spring on a mountain. The players take turns drawing from these tiles and playing followers on them as normal, ensuring that the water on each tile links to the water on the next. The water ends farms in the same way a road does, and followers may not be placed on the water itself. The lake is always played last. Some players have noticed this expansion causes larger farms to appear by the end of the game, one at each end.

[1] - BoardGameGeek page on the River mini-expansion