There is another river in the Carcassonne region. Like the first river, you can use these 12 tiles to start a Carcassonne game instead of the normal start tile. But, there is more. It can be used with the other river, to make an even longer river to start the game.
It has a branch, so you get the effect of two rivers instead of one. Its tiles have some of the features of the larger expansions: Inns & Cathedrals, Traders & Builders, and Princess & Dragon.
As always, you need the original Carcassonne to play with it, but you can use bits from the other expansions, too, if you like!