This is a two player river building / tile placement game. The object is to complete the canal in a loop. There are 16 identical tiles in the game, each with a straight section of canal on one side and a bend on the other. All the tiles are available for either player to use.

If during the course of the game, one player notices that the canal is now impossible to complete, they say "impossible" to the challenged player (who placed the last tile), who now must try to finish the canal alone. If the canal is completed the challenged player has won, if not, the challenger has won!