Avulsion and anastomosis in the panhandle region of the Okavango Fan, Botswana

Abstract

GPS water-surface elevation data from the panhandle region of the Okavango Fan (northwestern Botswana) show that a prominent anastomosed reach occurs within a relative valley-gradient depression inferred to represent a small graben structure. The Okavango River undergoes an abrupt change in pattern as it approaches the upstream fault zone of the graben, developing small meander loops and higher sinuosity in response to increased valley slope induced by faulting. Four channels split from and ultimately rejoin the Okavango trunk channel to define the anastomosed reach. The oldest (Filipo channel) diverted from the Okavango on the upstream horst block, rejoining the trunk channel 26 km downstream within the graben. The younger channels B, C and D all developed subsequent to the Filipo avulsion by forming linkages between the Okavango trunk and floodplain tributaries that had evolved by headward extension and incorporation of older abandoned channel segments. Diversion of the three younger anastomosed channels was likely abetted by increased aggradation of the trunk channel following down-faulting. Neotectonic movement, although involving only slight displacement, is seen to be the underlying cause of avulsion, anastomosis, and certain channel pattern changes in the Okavango panhandle. This contrasts with other parts of the Okavango Fan system where avulsion appears to result more from autogenic in-channel sedimentation conditioned by various roles of vegetation.