Typically, such patterns develop in basins with gentle slopes where the underlying geological structure is fairly homogenous and stable, exerting little or no control on river morphology. In marked contrast, the tributary networks of the Salween, Yangtze, and particularly the Mekong, are complex with different sub-basins often exhibiting different, and distinct, drainage patterns. These complex drainage systems have developed in a setting where the underlying geological structure is heterogeneous and active, and is the major factor controlling the course of rivers and the landscapes they carve out.