Contrasting styles of ephemeral river systems and their interaction with dunes of the Skeleton Coast erg (Namibia)

Abstract

The Skeleton Coast erg forms a prominent NNW trending dune belt, 6– 22 km wide, comprising dunes up to 50 m high, sub-parallel to the South Atlantic margin of Namibia. To a variable degree along its strike, the dune belt dams west-southwestward flowing ephemeral river systems on their route towards the Atlantic Ocean. The southern rivers, Koigab, Uniab and Hunkab, affecting the erg, are characterized by short, infrequent ephemeral flows from restricted catchments, whereas northern rivers, Hoanib and Hoarusib, are fed by large catchment areas favouring higher discharge and more frequent flows as the intertropical convergence zone and greater likelihood of monsoonal rainfall influence is approached. The potential for river damming increases in the same downwind direction through the erg because of increasing dune belt width and height and the change from low barchanoid dune forms in the south to large composite transverse dunes in the north. Whereas cross-sections of the southern part of the erg show considerable asymmetry with low barchans and sinuous crested dune forms eastwards but a high dune wall at the western edge, cross-sections over the northern erg are more uniformly symmetrical. As a consequence, the northern rivers are effectively dammed at the eastern erg margin, causing flood reservoir basins there. In contrast, southern rivers are dammed within the erg as the most pronounced and commonly encountered barrier is provided by the dunewall at its western border.