Geomorphology of Ephemeral Streams in the Kobo Basin

Abstract

The structural basin of Kobo (northern Ethiopia) is characterised by a semi-arid, monsoon-affected climate, and it is drained by ephemeral streams. The basin is physiographycally asymmetric with the highest mountains along the western margin where the largest rivers originate. The river morphology well matches the ideal model proposed by Schumm (The fluvial system. Wiley, New York, 1977), consisting of the headwater, the main trunk channel and the distributary system which represents the river terminus where the whole of the water flow vanishes due to infiltration and large amounts of sediment are deposited. The main geomorphic characteristics of the main stem and the distributary systems are described. The study river streambed is flat, devoid of any bedforms and horizontal and planar lamination is by far the most common sedimentary structure. Though in a few study reaches the Froude number calculated for bankfull discharge results steadily around one, i.e. in agreement with the extensive occurrence of the upper plane bed, the occurrence of outsized particles standing on or protruding from the streambed, travelling for long distances on the bed surface and showing no flow perturbation in the fine sediment in their vicinity, is interpreted by a new model of division association capable of explaining the typical division association observed in the study river deposits. It is characterised by large boulders rooted in a core coarse layer and the ubiquitous occurrence of horizontal lamination is interpreted in terms of vertical distribution of shear stress, hyperconcentrated flow and traction carpet processes.