The theoretic and methodological research of map generalization is an important field in cartography and geographical information systems. The difficulties of map generalization lie in selectively omitting some features while maintaining the overall characteristics of their spatial distribution. Simplifying drainages is an important aspect of map generalization.Structural patterns and density differences are salient characteristics of drainages, which should be maintained in generalized versions. This paper proposed a new method to generalize dendritic drainages while maintaining the density differences between sub-drainages, which is based on the layered division of the number of retained rivers in river trees. We analyzed the layered structure of dendritic drainages and their density differences, and found the density differences inside a drainage come from the differences of the number of tributaries of its sub-drainages. A method to allocate the number of retained rivers is accordingly proposed, which divides the overall retained number of sub-drainages of a drainage according to the ratio of the number of the tributaries. The structured drainage generalization is executed in three steps. Firstly, the river entities and river trees are constructed, and the number of tributaries for each river is counted. Then the number of rivers to be retained in the generalized version is computed according to the Square Root Law. Lastly the number of retention is divided among all of the drainages according to the number of their tributaries, the mainstream of a drainage with big length and spacing is selected, and the allocation and selection are recursively executed in the lower layer until the number of the selected rivers reaches the limit of the drainage. The method was implemented as plug-ins in Java Environment. Experimental results are compared with the hand-made generalized maps. A case study of drainage generalization showed that the method created acceptable results and the density differences among drainages were maintained effectively.