THE GEOMORPHOLOGY OF THE NYL RIVER AND FLOODPLAIN IN THE SEMI-ARID NORTHERN PROVINCE, SOUTH AFRICA

Abstract

ABSTRACT The Nylsvlei wetland is located on the 240 km2 floodplain of the lower Nyl River in the semi-arid Northern Province. The wetland is supplied seasonally with water and sediment by the Nyl and other small tributaries but has been little studied geomorphologically. The Nyl floodplain has formed within a basinal to synclinal structure on the downthrown side of a fault and the variable stratigraphy influences both ground and surface water flows, and the resultant fluvial landforms and ecology. Owing to downstream decreases in discharge and channel-bed gradient, the Nyl and several other tributaries presently decrease in size downstream and disappear at the margins of the floodplain, such that summer flooding occurs primarily as sheetflow. Thin clay layers deposited by these sheetflows effectively seal the floodplain surface, prolonging inundation and limiting groundwater recharge. Some groundwater recharge, however, occurs by floodwater infiltration through the channel beds of gravelly sand at the floodplain margins. Groundwater supply may contribute to growth of the numerous elevated, circular, floodplain islands which are characterised by woody fringes but sparsely-vegetated interiors. At the downstream end of the floodplain, bedrock outcrop induces convergence of the rare floodwaters and, together with a steepening of gradient, results in a well-defined channel again forming (Mogalakwena River). Correct identification of the factors giving rise to such wetlands, and the hydrogeomorphological processes governing their development, provides an essential context for long-term ecological studies as well as information for the design of effective management guidelines for these fragile habitats.