Fluvial suspended sediment transport and mechanical erosion in the Maghreb ( North Africa ) *

Probst & Amiotte Suchet (1992) are to be complimented on their valuable contribution to the investigation of erosion in the Maghreb (North Africa). As noted by the authors, the study of erosion at the global scale has neglected the Maghreb region and their recent paper draws attention to the intensity of sediment loss in this area. Their statistical analysis of a large body of suspended sediment yield data emphasises the complex multivariate nature of mechanical erosion and produces an estimate of the mean sediment yield of 420 t km" year. As indicated in their investigation, rock erodibility is a key factor in the generation of sediment yield but at the same time it is a difficult variable to quantify. Whilst the work of Chorley et al. (1984) may provide the basis for a coarse assessment of the relative resistance of a variety of lithologies from the USA it must be noted that similar lithologies may have quite different susceptibilities to erosion due to their physical and chemical make-up and past denudational history. For example, granites of the Askaouen sub-basin of the Souss river in southern Morocco have well developed gully systems and appear to be more erodible than adjacent areas of schists and volcanic lithologies (Moore & Fox, in preparation (b)). In addition many workers have drawn attention to the need to quantify the proportion of each basin underlain by different lithologies since only a small percentage of a basin may actually be contributing sediment. This is especially the case for large drainage basins, but even in moderate sized basins it may be significant. For example, in the Issen Basin of the High Atlas, Morocco, with an area of 1250 km, studies associated with the construction of the Barrage Abdelmoumen (Service Forestier d'Agadir, 1982) showed that over 50% of the volume of material reaching the reservoir was supplied by less than 10% of the catchment. Probst & Amiotte Suchet provide a multiple regression model of sediment yield which includes a number of the factors involved in the erosion