Vegetation, hydrology and sedimentation processes as determinants of channel form and dynamics in the northeastern Okavango Delta, Botswana

Summary The relationships between vegetation, hydrology, and the landscape-forming processes of erosion and deposition are examined in channels of the northeastern Okavango Delta, Botswana. Channel development appears to be due to the confinement of rapid water movement by vegetation processes and peat formation in backswamp areas. The channels have sandy beds and densely vegetated banks composed of erosion-resistant peat deposits between 1.5 m and 4 m thick. Sediment introduced into the delta is transported as bed-load, is unable to escape from the peat-lined channels and is therefore deposited along the channel floor. Channels that receive their water supply directly from source areas are therefore dominated by depositional processes. In contrast, channels that receive their water supply as overspill from these aggrading source channels do not receive bed-load sediments from source areas, and their beds tend to be erosional. Channel switching is therefore suggested to be due to a combination of erosional and depositional processes, rather than simply the result of depositional processes as has previously been suggested. A conceptual model of channel development and change within the northeastern Okavango Delta is presented. Resume On etudie les relations entre la vegetation, l'hydrologie et les processus de formation du paysage par l'erosion et la redeposition dans des canaux du nord-est du delta de l'Okavango, au Botswana. La formation des canaux semble etre conditionnee par la limitation imposee au mouvement rapide de l'eau par la vegetation et par la formation de tourbe dans les zones marecagueses. Le lit des canaux est sableux et comprend des bancs de vegetation dense composes de depots de tourbe d'un metre cinquante a quatre metres d'epaisseur, qui resistent a l'erosion. Les sediments qui sont amenes dans le delta sont transportes sur le fond, ne peuvent franchir les canaux longes de tourbe et se deposent done le long des canaux. Les canaux qui recoivent leur apport d'eau directement des regions des sources sont pour cette raison domines par les processus de deposition. Au contraire, ceux dont l'eau provient des excedents des premiers, ne recoivent pas de sediments de la region des sources et leur lit tend a s'eroder. Le deplacement des canaux semble donc devoir etre attribue a une combinaison des processus d'erosion et de deposition plutot qu'a la seule deposition comme on le croyait jusqu'alors. On presente un modele conceptuel de developpement et de transformation des canaux du nord-est du delta de l'Okavango.