Rola opadów w morfodynamice koryt rzek epizodycznych w górach wysokich strefy suchej (na przykładzie doliny górnego Dades, Atlas Wysoki, Maroko)

Rainfall in the arid zone is a very important factor responsible for changes in topography. Despite small amount of annual rainfall, single events can cause flash floods. The aim of the research was to determine the effects of precipitation of a given total and intensity on the dynamics of episodic river channel beds in the high mountains of the arid zone. The research was carried out in 3 catchments in the upper Dades basin located on the southern slopes of the High Atlas in Morocco. Rainfall data comes from three meteorological stations arranged next to experimental catchments. The results confirm the crucial role of precipitation in the morphodynamics of the mountain channels of the arid zone, as compared to other features of the natural environment. High intensity rainfall (max. 40.4 mm · h –1) lasting at least 1 hour is particularly important. Large change of the episodic river channel beds was also found during long (several days) rainfall of low intensity but high total (max. 83.6 mm). This is a result of low retention due to the low density of plant cover and the thin regolith. The transport of the material in the tested channels took place after a rainfall of 11.8 mm in total and a maximum intensity of 5 mm · h –1. Boulders (> 80 cm) were transported only in the case of rainfall events with a total of approx. 70 mm and intensity over 40 mm/day. Based on the analysis of: (i) rainfall intensity and total, (ii) flow rate, (iii) transport length of the channel material, and (iv) sediment budget, three types of morphogenic rainfall were distinguished. As many as 57 % of them were classified to type B, i.e. with a total of less than 30 mm and intensity above 4 mm · h -1, 33 % to type C, i.e. with a total of more than 30 mm and intensity above 4 mm · h -1, and only 10 % to type A, i.e. with a total of less than 30 mm and intensity below 4 mm · h-1. The lowest impact on channel morphology was the result of type A of rainfall that caused only local changes in the channel bed and short-distance transport of pebbles. The most common type of rainfall – B caused partial reworking of channel bed and long-distance transport of channel material of size up to cobbles. The largest changes in channel 2 Elżbieta Rojan, Maciej Dłużewski morphology expressed by full reworking of channel bed and transport of coarse material including boulders results from type C.