Drainage network morphometry and evolution in the eastern Lesotho highlands, southern Africa

Abstract

Present-day drainage patterns in mountain regions are strongly affected by antecedent tectonic and geologic factors. In the mountains of eastern Lesotho, southern Africa, bedrock comprises flat-lying Jurassic basalts which have given rise to a relatively uniform initial land surface. Here, we present key morphometric measurements of river drainage patterns from this region, from six 4th order river basins, including principal river sinuosity, river long profiles and valley cross profiles, in order to evaluate geological controls on drainage evolution and the potential ages of river elements. Results show that there are highly variable spatial patterns of river morphometric properties, even between adjacent basins, despite identical geologies and altitudes across the region. However, northerly catchments show knickpoints, variable long profiles and have steep sided and V-shaped valley cross profiles. Southerly catchments have much smoother long profiles, no knickpoints, and concavo-convex valley cross profiles. There are also differences in river sinuosity values, both for the catchments as a whole and for reaches of different lengths within the catchments. These spatially varying river morphometric and drainage properties most likely are the long-preserved outcomes of epeirogenic forcing on the land surface (including tectonic uplift, mantle relaxation and rebound during the Cenozoic), and subtle long-term variations in climate from north to south across the region, including in precipitation and weathering.