Geomorphology of Eritrean River Systems

Abstract

Eritrea is a mountainous country. Its physiography and landscape are the results of uninterrupted uplift and faulting in the Cenozoic. Both the highlands and the rift escarpment are characterised by a high relief ratio, the occurrence of deeply incised rivers systems and rugged geomorphology. Rivers in the headwaters are incised into bedrock or may have the streambed covered by a thin accumulation of bed material. Notwithstanding the steep gradient, several bedrock channels with alluvium show a meandering pattern. Such meandering pattern is unexpected and poorly studied in a mountain environment. A relatively small set of meander parameters was measured to investigate processes generating meanders in a steep bedrock channel and to compare the bedrock curves geometry to lowland, alluvial meandering rivers. The data showed that both steep bedrock and flatter alluvial meanders share very similar geomorphic characteristics. Though it is generally recognised that tectonics may have influenced the shape of the longitudinal profile of a river, few studies were conducted on this topic in an area with a very high uplift rate such as Eritrea. The downstream variation of the stream gradient index (SGI) was obtained from Google Earth for some rivers representative of different physiographic areas of Eritrea. The data indicate that the SGI is sensitive to recent tectonics, whereas past uplift is hardly recorded in the river profiles, which may have reached equilibrium condition in a shorter time than expected.