Aspects of floodplain deposition in semi-arid ephemeral rivers, examples from the Kuiseb river valley, central Namibia

Abstract

Topographic and sediment work in the Kuiseb river, currently an ephemeral stream in central Namibia, has shown that floodplain deposition took place mainly downstream from zones of significant external sediment input. Two floodplain types are identified as floodplain islands which developed as braided river deposits in the midstream section of the river, and infill deposits developed from anastomosing streams in the downstream section. The two types are dependent on the geometry of the river cross section and the amount of sediment available. The topography, sediment content (fine sand and silt units) and structure all imply the deposition of floodplain deposits under wetter conditions. Particle size analysis of floodplain silts in relation to river bed and sand dune silt, indicates a closer association between the floodplains and Quaternary Homeb deposits. These data suggest that the floodplains likely formed under locally wet conditions which eroded the Quaternary Homeb deposits during the early-mid Holocene.