The Rhône valley separates them from the Chablais Alps in the west and from the Pennine Alps in the south; the upper Rhône valley separates them from the Lepontine Alps to the southeast; the Grimsel Pass and the Aare valley separates them from the Uri Alps in the east, and from the Emmental Alps in the north; their northwestern edge is not well defined, describing a line roughly from Lake Geneva to Lake Thun. The Bernese Alps are drained by the river Aare and its tributary the Saane in the north, the Rhône in the south, and the Reuss in the east.