This paper studies the evolution of the Ara river from 1929 till now in a bench between Boltaña and Aínsa. Methods are based mainly infield work and in the use of aereal photographs corresponding to 1929, 1946, 1956, 1981, 1984 and 1988. The authors have elaborated a geomorphological cartography that allows us to establish the changes in channel morphology: progressive enlargement ofthe fluvíal bed; predominance of areas without plant cover until 1956; development of plant cover from 1956 unta now; subdivision of the main bars in small secondary bars owing to a increase of the number of channels; increasing human activities from 1974 (building of the Mediano reservoir and channelization of the river by means of lateral dykes).