Fluvio-alluvial source-sink relationships at the Skeleton Coast of northern Namibia

Abstract. The hyperarid Skeleton Coast of Namibia hosts a diverse suite of alluvial landforms, often shaped by varying degrees of lateral and distal confinement primarily through fan coalescence, the Atlantic Ocean, and the Skeleton Coast Erg. Considerable heterogeneity also characterises the source areas, as reflected in catchment morphometry, lithology, and coast-perpendicular moisture gradients, where larger inland-draining catchments intercept more precipitation. On the regional scale, this heterogeneity blurs source-sink relationships between alluvial landforms and catchments, despite overall geomorphic drainage maturity which could imply similar efficacy in source-sink communication.

To disentangle these patterns, we mapped 67 drainage systems and obtained a dataset including (hydro-) morphometric, climatic, and geologic parameters for these systems. An exploratory data analysis framework combining cluster and partial correlation analysis was applied on these datasets. Three distinct clusters were identified for both alluvial landforms and catchments, which match only weakly. The clearest source-sink coupling is attributed to a cluster of near-coastline fans, spanning much of the study area but with a spatial focus on its northern portion. By contrast, the majority of alluvial landforms form bajadas strongly influenced by the Skeleton Coast Erg, where distal confinement masks simple morphometric scaling.

Our results highlight fan confinement as the main driver affecting source-sink relationships at the Skeleton Coast. Fan morphometry appears to be more decisive than catchment properties, with fan gradient (<1° on average) emerging as a reliable discriminator. No robust climatic or lithological control on fan morphometry could be identified, although Sentinel-1 radar backscatter indicates more stable fan surfaces south of the Skeleton Coast Erg.

Overall, the Skeleton Coast represents a low-dynamics dryland margin where fan gradient provides the most meaningful parameter for source–sink analysis. Least-confined fans show the strongest coupling and thus constitute promising targets for paleoenvironmental reconstruction, while environmental conditions in the southern portion of the study area may have so far provided the most favourable conditions for long-term archive preservation.