The 1800‐km2 Okavango alluvial fan of northern Botswana represents an unusual depositional setting in which peat‐forming perennial swamps (6000 km2) occur in a region of aeolian and semi‐arid sedimentation within an incipient graben of the East African Rift. A channel system distributes water and sediment on the fan surface but cannot contain seasonal flood water, which spreads laterally from the channels through permeable channel margins, sustaining the flanking swamps. All sediment introduced is deposited on the fan. A detailed study of sediment movement and associated hydrological conditions in the channels was undertaken to examine sediment dispersal. Bedload greatly exceeds suspended load (at least by a factor of four). Vegetation and peat form permeable levees which confine the channels. In the upper reaches, two‐way exchange of water occurs between channel and swamp depending on the season, but on the fan itself, channels lose water to the swamp. Bedload measurements reveal that the channel system is in a state of grade disequilibrium, with interspersed depositional and erosional reaches. Deposition of most of the incoming bedload occurs on the upper portion of the fan in a meandering and anastomosed channel system, but on the midfan, deposition of bedload occurs by channel‐bed aggradation, at a rate of up to 5 cm yr–1. Further down slope, the channel enters a large lake where all remaining bedload is deposited. The presently observed sedimentation patterns may be due to a recent disturbance of the fluvial system, either by avulsion or neotectonics.
Summary
The progressive abandonment and desiccation of the lower Nqoga River system and its associated vegetation‐covered peat deposits, has resulted in the development of subsurface peat fires. These appear to have been burning within the study area for decades and consist of a mosaic of burning fronts, the first of which burns to a depth of up to 15 cm. This relatively shallow fire destroys the bases of stems and plant rhizomes, thereby preventing re‐establishment of the former plant community by vegetative growth. Deeper fires are initiated where cracks develop in the peat during drying and desiccation. The depths to which these burn are limited by peat moisture content or by the presence of an incombustible inorganic substrate. The initial collapse is some 30–40 vol. %. Further compaction is brought about by rainfall and animal trampling which may cause volume reductions of as much as 98%.
The short‐term consequences of subsurface peat fires include the release of plant nutrients, enhancement of soil texture and soil water retention properties, and temporary transformation of the area into a terrestrial habitat. However, physiographic changes induced by the peat fire result in its eventual reflooding. It is proposed that the time delay between burning and reflooding, and the rate and depth of reflooding, determine the nature of the habitats which form after re‐inundation. These processes are important in maintaining the overall structure and functioning of the Okavango Delta ecosystem.
Résumé
L'abandon progressif et le dessèchement de la basse Nqoga et des dépôts de tourbe qui y sont associés ont abouti au développement de feux de tourbe sous la surface. II semble que ceux‐ci brûlaient depuis des décennies à l'endroit des recherches et qu'ils se composent d'une mosaique de petits feux dont le premier brûle jusqu'à une profondeur de 15 cm. Ce feu assez superficiel détruit les bases des tiges et les rhizomes des plantes, empêchant de ce fait le rétablissement de la plante originelle par simple croissance végétative. Des feux plus profonds surviennent lorsque des fissures se développent dans la tourbe en voie de dessication. La profondeur à laquelle ils brûlent est limitée par l'humidité de la tourbe ou par la présence d'un substrat inorganique incombustible. l'effondrement initial correspond à quelque 30 à 40% en volume. Les chutes de pluies et le pietinement des animaux peuvent tasser encore plus et causer des réductions de volume atteignant 98%. Les conséquences à court terme des feux de tourbe souterrains comprennent la libération de nutriments végétaux, une amélioration de la texture du sol et de ses propriétés de rétention d'eau, et la transformation temporaire de la région en un habitat terrestre. Cependant les modifications physiographiques induites par les feux de tourbe résultent ultérieurement en de nouvelles inodations. On pense que le délai entre le feu et l'inondation ainsi que l'importance et la prondeur de Pinondation déterminent la nature des habitats qui se forment après inondation. Ces process...
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