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Le présent article se propose de faire le point sur les connaissances actuelles quant au rôle de la végétation dans la protection contre l'érosion hydrique de surface. Il expose, dans un premier temps, les mécanismes d'action de la végétation vis-à-vis des processus d'érosion et de sédimentation. Dans un deuxième temps, il présente des résultats de recherches sur l'efficacité des différentes formations végétales et l'importance du taux de couverture végétale pour la lutte contre l'érosion de ravines. Pour citer cet article : F.

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The cross -sections of the river El Akarit are of considerable importance because they explain the geomorphological and palae-climatical evolution of south-Tunisia for 40.000 years. The present study has permitted us to distinguish the succession of two sedimentary sequences separated by an unconformity formed by the channeling action of running waters and already observed by W.D. Page (1972). Thanks to a multidisciplinary collaboration successive environments of these two sequences have been studied in detail. The first, at the end of late Pleistocene begins with a wet period whose age is uncertain in spite of carbon 14 dating tests and the presence of «mounsterian» levels. The microfauna especially the Foraminifera indicate variable salted ponds with relation to probable transgression of the sea (about 27.000 BP ?). The second sedimentary sequence also begins with hydromorphic deposits, more typical of euryhaline environments with thalassoid affinities according to the microfauna, the Molluscs and probably the microflora. This period is between 8700 and 7400 BP. Here also the pollens indicate a dominant steppe-like vegetation with a slight increase of trees at about 6500-6000 BP, just before the general deposits of gypsum-dusts and the beginning of a sebkha period. The resumption of stream waters and the deposit of loess indicate the end of the extreme dry period and the return to a steppe-like vegetation nearly 3500 years ago. For 2 or 3 millennial even when the marine level remained high, we noted a very speedy gullying of about 8 to 10 m, along this valley. In addition to this paleoclimatic reconstitution this study calls into question the interpretation and the datation of glacis 2-the major shape of south-Tunisian piedmonts-and chiefly the assumption of the tectonic stability of the Oudref threshold between the chotts and the gulf of Gabès.

À la lumière de recherches récentes, l'hydrographie du Bas-Sahara est revisitée. Il est montré que les oueds mythiques, Igharghar à partir du sud du Grand Erg Oriental, Mya au niveau de Ouargla et Rhir n'existent pas. Parmi les oueds réels fonctionnels, on commence à mieux connaître ceux qui descendent de l'Atlas saharien avec leurs barrages et beaucoup moins bien ceux de la dorsale du M'Zab. Des oueds réels fossiles viennent d'être découverts dans le Souf, à l'amont du Grand Erg Oriental. Les seules vraies rivières, pérennes, tel le grand drain, sont celles alimentées par les eaux de collature des oasis et des réseaux pluviaux des villes.
New researches on Bas-Sahara hydrography have been performed. They show that mythical wadis such as wadi Igharghar from south of the Grand Erg Oriental, wadi Mya close to Ouargla and wadi Rhir do not exist. Among the functional real wadis, those that run from the saharan Atlas mountains are best known, due to their dams. Those of the M'Zab ridge are still poorly studied. Fossil real wadis have been just discovered in the Souf region, north of the Grand Erg Oriental. The only genuine perennial rivers, such as the grand drain, are fed by oasis collature waters and towns rainwater networks