Incremental aggradation on the Okavango Delta-fan, Botswana

Distributary channel switching is a relatively frequent event in the upper reaches of the Okavango Delta-fan, Botswana. This phenomenon was investigated by means of detailed topographic surveys and excavations along an abandoned channel and depth probing along an active channel system. This work has confirmed that channel switching is the result of aggradation within the channel systems. Initially, new channel systems are erosive but later in their evolution both channel bed and adjacent swamp (peat) areas begin to aggrade. This phase leads to a change in channel gradient and causes the channel to become moribund. Abandonment follows with consequent desiccation of the peat. Burning of the peat completes the cycle of the channel evolution, the entire process taking about one hundred years under present flow conditions. The intial aggradation phase results in the accumulation of about 4 m of peat, but net aggradation after collapse of the peat following a peat fire is 30-40 cm. channel beds probably also experience about 4 m of aggradation, as measured from the eroded, channel floor. After a peat fire, the channel bed sands remain elevated by about 1 m relative to the surrounding, burnt out peat. As a result of the collapse of the peat, the abandoned area becomes available for reflooding.