The sedimentary dynamics of active fluvial channels on the Okavango fan, Botswana

Abstract

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.

Unusual development of sandur sedimentary succession, an example from the Pleistocene of S Poland

“…The upper plane bed and alternating side bars are common depositional forms in anastomosing channels (Rust & Legun, 1983;Wende & Nanson, 1998; Tooth & Nanson, 1999). The grain size of these deposits (fine to coarse sand) is also similar to that of alluvium from anastomosing rivers (McCarthy et al, 1991;Smith et al, 1997). The presence of two facies -the main and the secondary channels -obviously adds weight to this environmental interpretation (cf.…”

“…The sandur fits with its interpretation as an anabranching channel system with low-energy discharges. The moderate sinuosity of the palaeochannels corresponds with the morphology of most anastomosing rivers (see McCarthy et al, 1991;Teisseyre, 1992;Smith et al, 1997;Makaske, 1998). The estimated depth of the first-rank palaeochannels is in agreement with the observations of McCarthy et al (1991) and Smith et al (1997).…”

Fluvial geomorphic elements in modern sedimentary basins and their potential preservation in the rock record: A review

“…Of these large DFS, the Okavango DFS has been widely studied (e.g., McCarthy et al, 1988McCarthy et al, ,1991McCarthy et al, , 1992 Axial DFS occur where a half-graben is filled and the axial system spills into an adjacent (downstream) 482 half-graben if accommodation exists between the lateral DFS (Mack et al, 1997(Mack et al, , 2006. This fill and spill 483 type scenario is considered to have occurred a number of times, resulting in linkage of half-grabens to 484 form a throughgoing axial river system thatfeeds an axial DFS in the terminal half-graben (Mack et al, 485 1997).…”

The spatial and temporal characterisation of flooding within the floodplain wetland of the Nyl River, Limpopo Province, South Africa

“…Water accumulation within the floodplain wetland of the Nyl River during 1995-1996: (a) 27 October 1995;(b) 12 November 1995;(c) 28 November 1995 (d) input from the tributaries on the north-western side of the floodplain, particularly from those to the north of Mookgopong (e.g. Tobiasspruit and Andriespruit).…”