Stream channel development in the southern parts of the High Atlas Mountains, Morocco

Abstract

The aim of this study is to review current knowledge on channels development in mountain basins in arid zones. The research was intended to describe the main triggering factors responsible for the development of ephemeral river channels in such areas. A detailed study was made of the Upper Dades basin located on the southern slopes of the High Atlas Mountains (Morocco). In this paper we analyse the morphometry of channels of different orders in three small basins chosen for a thorough study in order to present the details of channel characteristics. The results show that the development of stream channels of the same order may vary greatly. This is mainly due to variation between the basins in lithology, vegetation cover, precipitation amounts and hillslope sediment supply. We prove however that the key agents of stream channel development are the energy of water and associated mass gravity movements in the period when water is absent. The morphological component provides strong evidence for this conclusion. The results show that runoff energy generally compensates for the low amount of water in 2nd and 3rd order channels where erosion is dominant. The 4th order channels are therefore better adapted to evacuate significant discharges (weak slope, large channel) which therefore decreases the erosive capacity of their stream flows. In this paper we also discuss whether the impact of the limited hillslope vegetation and the supply of coarse-grained sediment in the channels are important in the development of channels in mountain basins in arid regions. We conclude that intense rainfall-runoff events that lead to short-lived but high-energy flash floods have a more significant impact in contemporary channel development. Channels, especially those of 3rd and higher order, tend to be very unstable during large floods. We show that channels located in small basins in mountains in arid zones are in a state of almost permanent non-equilibrium.