Drainage pattern and regional morphostructure at Melka Kunture (Upper Awash, Ethiopia)

autorisation des éditeurs de l'ouvrage par email du 01/07/05Melka Kunture depression belongs to the upper Awash basin. A village of the same name stands on the border of the Ethiopian Plateau, some fifty kilometres south of Addis Ababa. The basin surface is around 3000 km2 and its altitude varies from 2000 to 2050 m. It is delimited by pliocene volcanoes the main ones being Wachacha and Furi in the north, Boti and Agoiabi in the south. Upstream, the basin is a collapsed plain and downstream a less collapsed zone of enclosed gorges. Its eastern limit is marked by the main graben of the Ethiopian Rift belonging to the large East-African rift system. The Melka Kunture area is made up of valleys with inner terraces of which have resisted erosion. The visible thickness of those deposits is around 30 m, but the total thickness of the various levels is about 100 m. The Melka Kunture area shows dissimilar morphology between the right and left banks of the Awash River. On the left bank, the plain goes up to the Wachacha slopes as rather regular glacis, while the right bank shows a series of steps linked to a compartmentation formed by a Northeast-Southwest fault system. This segmentation affects the series of less collapsed compartments separating the Melka Kunture region from the rift graben. Tectonic evolution of the Melka Kunture area determined the sedimentation and the morphological types. This evolution still develops today. The Holocene topography, regularised in wide glacis, is in complete imbalance with flows responsible for vigorous incisions and fast destruction of the pedological and vertisolic cover. A morphostructural analysis based on topographic maps was used. This technique considers base maps as useful objects that can reveal quite subtle morphology. Its aim is to extract the best possible information contained in cartographic documents, which must be the basis of any geographical, geological or geomorphological study