The drainage pattern of a river network is the arrangement in which a stream erodes the channels of its network of tributaries. It can reflect the geographical characteristics of a river network to a certain extent, because it depends on the topography and geology of the land. Whether in cartography or GIS, hydrography is one of the most important feature classes to generalise in order to produce representations at various levels of detail. Cartographic generalisation is an intricate process whereby information is selected and represented on a map at a certain scale, not necessarily preserving all geographical or other cartographic details. There are many methods for river network generalisation, but the generalized results are always inspected by expert cartographers visually. This paper proposes a method that evaluates the quality of a river network generalisation by assessing if drainage patterns are preserved. This method provides a quantitative value that estimates the membership of a river network in different drainage patterns. A set of geometric indicators describing each pattern are presented and the membership of a network is defined based on fuzzy logic. For each pattern, the fuzzy set membership is given by a defined IF-THEN rule composed of several indicators and logical operators. Assessing the quality of a generalisation is done by comparing and analysing the value before and after the network generalisation. This assessment method is tested with several river network generalisation methods on different sets of networks and results are analysed and discussed.