Morphodynamics of small high-mountain wadis in arid zones (on the examples of the tributaries of the upper Dades River, High Atlas Mountains, Morocco)

The paper reports on a study of the impact of morphology and lithology of small catchments on episodic river channel development in the southern High Atlas Mountains in Morocco. A detailed analysis focused on three catchments of small wadis featuring different relief and lithology along the upper, middle and lower course of the Dades River. The fieldwork combined with an analysis of topographic and geological maps and of satellite images concluded that the development of the river valley and channel structures was related to the main stages in the development of the landform in this part of the High Atlas. The channel dynamics were found to be typical of arid mountains, which was particularly apparent within a denudational-and-fluvial zone that reached up to 3,000 m a.s.l. The dominant morphodynamic system of this zone was pluvio-gravitational where episodic heavy rainfall events triggered fast displacement of stone mantle covers down the slopes and into the river channels. An analysis of the deposition zones in the lower channel reaches and on alluvial fans at the confluence of the side valleys and the main valley leads to the conclusion that the main factor, which determines the channel morphology is the impact of debris flows and heavily loaded streams and rivers. The deposited material is then transported outside the mountain zone and deposited within broad river channels. The development of the river channels in the three catchments depends on their order, morphometric property, slope lithology and the amount of material received from the slopes.