Vegetation, hydrology, and sedimentation patterns on the major distributary system of the Okavango fan, Botswana

Abstract

This study investigated the distribution and determinants of marsh vegetation along the major distributary channel system of southern Africa’s largest wetland, the Okavango Delta, using a large, inter-disciplinary data set. Eight communities were recognized, dominated respectively by Pennisetum glaucocladum, Phragmites mauritianus, Cyperus papyrus, Cyperus papyrus/Miscanthus junceus, Miscanthus junceus, Imperata cylindrica, Pycreus nitidus and a mixed bog community. The Pennisetum glaucocladum community is situated in the Panhandle (a narrow valley reach at the head of the fan) on elevated scroll bars that are flooded seasonally for short periods (days to weeks). The Phragmites mauritianus community occurs largely in the Panhandle where channel margins have high clay contents and where soils are seasonally flooded for moderate periods (months). Fires are widespread in the Panhandle, but both Pennisetum glaucocladum and Phragmites mauritianus are stoloniferous, and meristems occur below the soil surface and are protected from fire. In contrast, the rhizomatous sedge C. papyrus dominates in situations where meristems are permanently submerged, and therefore protected from fire, such as areas of open channel water where current velocities are sufficiently low to enable the extension of C. papyrus into the channel. This situation exists where channel avulsion has recently taken place or where discharge is reduced by water loss from the channel by overspill. The semi-floating habit of C. papyrus in the channel fringe results in high hydraulic conductivities, which promotes water loss from channels and leads to sediment deposition within channels. Miscanthus junceus occurs in areas where the nutrient status of water is low, seasonal changes in water level are small, and the average water level is approximately constant over decadal time scales. It occurs some distance from the channel on the upper reaches of the Delta and progressively closer to the channel downstream such that it is the dominant channel fringe species in the distal reaches. This pattern, where a community occurs progressively closer to the channel downstream is similar for communities dominated by Imperata cylindrica and Pycreus nitidus suggesting that environmental gradients (probably nutrient supply) perpendicular to the channel axis are steep and that they are mirrored by long-range environmental gradients downstream. An analysis of hydraulic characteristics of this distributary river system illustrates that channel width varies most with variation in discharge, while channel depth and current velocity are relatively constant over the range of discharges in the study area. Since channel width is primarily a consequence of vegetation processes in the channel margin, especially the growth of the giant sedge C. papyrus it is clear that channel hydraulics are affected largely by vegetation.