Variation in material transport and water chemistry along a large ephemeral river in the Namib Desert

1. The chemical characteristics of floodwaters in ephemeral rivers are litt le known, particularly with regard to their organic loads. These rivers typ ically exhibit a pronounced downstream hydrological decay but few studies h ave documented its effect on chemical characteristics and material transpor t. To develop a better understanding of the dynamics of floods and associat ed material transport in large ephemeral rivers, floods of the ephemeral Ku iseb River in south-western Africa were tracked and repeatedly sampled at m ultiple points along the river's lower 220 km. 2. We quantified the composition and transport of solute and sediment loads in relation to longitudinal hydrological patterns associated with downstre am hydrological decay. Source and sink areas for transported materials were identified, and the composition and transport dynamics of the organic matt er load were compared to those described from more mesic systems. 3. Concentrations of sediments and solutes transported by floods in the Kui seb River tended to increase downstream in association with pronounced hydr ological decay. The contribution of particulate organic matter to total org anic load is among the highest recorded, despite our observation of unusual ly high levels of dissolved organic matter. Hydrological decay resulted in deposition of all transported material within the lower Kuiseb River, with no discharge of water or materials to the Atlantic Ocean. 4. Our results suggest that longitudinal variation in surface flow and asso ciated patterns of material transport renders the lower Kuiseb River a sink for materials transported from upstream. The downstream transport and depo sition of large amounts of labile organic matter provides an important carb on supplement to heterotrophic communities within the river's lower reaches .