Wetlands in drylands: geomorphological and sedimentological characteristics, with emphasis on examples from southern Africa

Abstract

Wetlands are poorly documented features of many landscapes, and there is often little understanding of the geomorphological controls on their origin, development and characteristics. This paper addresses the apparent paradox of wetlands in drylands, focusing particularly on the geomorphology and sedimentology of wetlands in southern Africa. Drylands are characterized by high (but variable) levels of aridity, reflecting low ratios between precipitation and potential evapotranspiration, so wetlands can only exist where there are locally positive surface water balances for all or part of the year. Most moderate to large wetlands in drylands are thus maintained by river inflows that combine with other factors that serve to impede drainage or reduce infiltration, including faulting, rock outcrops, swelling soils, and ponding by tributary or aeolian sediments. Together with variations in sediment supply, vegetation communities, and levels of animal activity, this promotes a diverse range of wetlands that span a continuum from permanently inundated, to seasonally inundated, to ephemerally inundated. In detail, every wetland has a unique range of geomorphological and sedimentological characteristics but, at a general level, the dryland setting can be shown to impart some distinctive features. By comparison with humid region (tropical and temperate) wetlands, we propose that many wetlands in drylands are characterized by: 1) more frequent and/or longer periods of desiccation; 2) channels that commonly decrease in size and even disappear downstream; 3) higher levels of chemical sedimentation; 4) more frequent fires that reduce the potential for thick organic accumulations and promote aeolian activity; and 5) longer timescales of development that may extend far back into the Pleistocene. Additional studies of wetlands in different drylands may reveal other distinctive characteristics. Correct identification of the factors giving rise to wetlands, and improved understanding of the geomorphological and sedimentological processes governing their development, is vital for the design of sustainable management guidelines for these diverse yet fragile habitats.