Dry valleys (mekgacha)

Conventional rivers are absent from much of Botswana, with only the Okavango, Chobe and Zambezi systems in the extreme north containing perennial flowing water. Ephemeral rivers occur in the eastern hardveld, but the most extensive components of the surface drainage are the networks of fossil or dry valleys (termed mekgacha in Setswana and dum in various San languages) that cross the sandveld. This chapter presents the first holistic review of current knowledge about these enigmatic landforms. It does so using a range of evidence types, from radar remote-sensing to the analysis of historical documents written by missionaries and explorers. The chapter considers dry valley distribution, morphology, and contemporary and historical hydrology before discussing valley evolution over longer timescales. It concludes with a synthesis of the main arguments concerning how dry valley systems may have formed, including the balance between conventional fluvial incision and processes such as groundwater seepage erosion.