The site is situated in the Sfax Region, in the Gulf of Gabès, at about 25 km south-east of the town Mahrès, 34°22'N 10°18' E. From a geomorphological point of view Kneiss can be considered as the estuary of El Melah wadi and Smara wadi. However, these wadis (ephemeral streams) contain water only during and after winter rainfall and therefore no brackish transition zone is present. Inland these wadis are narrow, but towards the coast they widen to a huge and almost flat extensive plain delta with saltmarshes, sebkhas and some sand dunes. The delta plain of Smara wadi is bordered by sandy hills with olive plantations. It develops gently into the extensive intertidal flats, the former estuary now has been washed away by the rising sea level, leaving five silt-sandy islands (Dzirat el Bessila, also named Kneiss, the largest one, Dzirat El Hajar, Dzirat El Laboua and Dzirat El Jazirat el Rhabia) covering an area of 5,850 ha. Extensive intertidal mudflats cover the area between the high and low waterline and are intersected by deep (> 10 m) drainage channels, streams and branches which are connected with the wadis. Most of the intertidal flats are covered with eelgrass Zostera sp. Above the high waterline salt marshes and sebkahs occur, which can be flooded during spring tides and strong onshore winds. The sebkhas consist of bare mud with a salty crust when dry. Temporary, brackish ponds can occur in the sebkhas after high winter rainfall, which usually dry out during spring. The tidal difference is up to 2.5 meters during spring tides and a few decimetres during neap tides. During spring tides all the intertidal flats and the lower, vegetated parts of the saltmarshes are flooded, as well as large parts of the sebkhas. The intertidal mudflats are the most important feeding areas for waders and other water birds. Some mudflats contain very soft mud, whereas other areas are more sandy and firm. The site is of international importance for hibernating birds, 70% of Tunisian avifauna. Moreover, 1% of total migrating bird population directed to or coming from the Sahara, crosses Kneiss Islands.
Dijk A. J. van, Dijk K. van, Dijksen L.J., Spanje T.M. van, Wymenga E. (1986) Wintering waders and waterfowl in the gulf of Gabès, Tunisia, January-March 1984.
Final Report of the WIWO Tunisia project 1984, WIWO report no. 11. Have T.M. van der, Baccetti N., Keijl G.O., Zenatello M. (editors) (1997) Waterbirds in Kneiss, Tunisia, February 1994, WIWO Report 54.