||Abstract. – La Ronde is a polysaprobic river located in the swiss Jura. The river flows both along a aerian way and through a karstic system. This situation made it possible to compare the self-purification capacity of both flows, with indicative values of water quality based on the benthic communities of diatoms. The ecomorphologic features of the aerial flow are described, in relation with their self-purification capacities. The selfpurification abilities of agitated, slow, not deep and dispersed flows are better than other types of flow. These features are compared with the hypothetical morphology of the karstic passages, which could promote catabolic processes.