AVULSION AND ANASTOMOSIS IN THE PANHANDLE REGION OF THE OKAVANGO FAN,BOTSWANA

GPS water-surface elevation data from the panhandle region of the Okav ango Fan (northwestern Botswana) show that a prominent anastomosed rea ch occurs within a relative valley-gradient depression inferred to rep resent a small graben structure. The Okavango River undergoes an abrup t change in pattern as it approaches the upstream fault zone of the gr aben, developing small meander loops and higher sinuosity in response to increased valley slope induced by faulting. Four channels split fro m and ultimately rejoin the Okavango trunk channel to define the anast omosed reach. The oldest (Filipo channel) diverted from the Okavango o n the upstream horst block, rejoining the trunk channel 26 km downstre am within the graben. The younger channels B, C and D all developed su bsequent to the Filipo avulsion by forming linkages between the Okavan go trunk and floodplain tributaries that had evolved by headward exten sion and incorporation of older abandoned channel segments. Diversion of the three younger anastomosed channels was likely abetted by increa sed aggradation of the trunk channel following down-faulting. Neotecto nic movement, although involving only slight displacement is seen to b e the underlying cause of avulsion, anastomosis, and certain channel p attern changes in the Okavango panhandle. This contrasts with other pa rts of the Okavango Fan system where avulsion appears to result more f rom autogenic in-channel sedimentation conditioned by various roles of vegetation.