THE FLUVIAL DYNAMICS OF THE MAUNACHIRA CHANNEL SYSTEM, NORTHEASTERN OKAVANGO SWAMPS, BOTSWANA

This study was undertaken to investigate the role and function of seco ndary channels (those that arise within the swamps and carry no extern ally derived bedload sediment) in the Okavango Delta, and focused on t he Maunachira Channel of the north-eastern swamps. Three reaches were identified: an erosive upper reach; a depositional middle reach; and a stable lower reach where no sediment movement is occurring and the ch annel bed is vegetated. The erosive reach is confined to the most prox imal section of the channel and is associated with inflow into the cha nnel from tributaries and from the surrounding swamp. Downstream of th is reach, the channel becomes aggradational. The aggradational reach i s associated with a marked rise in the channel water level relative to the surrounding swamps, which promotes water loss from the channel. S ome of the water leaked from the channel supplies the Mboroga Channel to the south. Cyperus papyrus grows vigorously along the aggrading rea ch, and partially constricts the channel. Aggradation results in a dec rease in channel gradient and flow velocity declines to below 0.4 m/s. , when bottom-rooted vegetation is able to colonise the channel bed, t erminating sediment movement. Channel margins in these lower reaches a re characterised by Miscanthus junceus, a non-invasive plant species, which promotes long-term stability. The zone of aggradation is migrati ng upstream at about 170 m/a. The study suggests that secondary channe ls arise due to local oversteepening of the regional gradient and thei r role is to redistribute sediment down this gradient.