The River adds 12 "River" tiles. These tiles are always picked and placed first when the game starts, and afterwards the game continues with the original rules.
Using "The River", each game of Carcassonne starts with a unique setup, instead of the standard setup with the intial tile. This mini-expansion is so much appreciated that in a lot of countries it is actually included in the base game box.
We'd also like to take advange of this news to tell you that the very annoying display bug that appeared on the Chrome internet browser some months ago has now been fixed by Google in Chrome version 68 (the tiles art images were misaligned). This bug was particularly annoying for Carcassonne players (but also with other games). So if you have Google Chrome version 66 or 67, we strongly advise you to upgrade your browser to version 68 or above to fix this problem.