The development of a subterranean drainage system depends on the way in which subterranean confluences between different rivers can be formed. Different from surface, in which confluences are determined by processes related to the surface runoff of water, in subterranean karst confluences have a random pattern and are related to certain circumstances independent of the underground flow. These conditions are: pre-existence of circulation ways and the way they are distributed in space. At these the peculiar processes of subterranean karst flow determined by the flow under pressure, the only one that can explain the systematic appearance of confluences, have to be added. In function of these parameters a morphogenetic classification of subterranean confluences is given.