THE ROLE OF CYPERUS-PAPYRUS L IN CHANNEL BLOCKAGE AND ABANDONMENT IN THE NORTHEASTERN OKAVANGO DELTA, BOTSWANA

Channel blockage and abandonment in the Okavango Delta has been consid ered to be caused by either a combination of encroachment of Cyperus p apyrus from the channel banks into the channels, or the development of papyrus debris blockages in the lower reaches of major distributary c hannels. This has been investigated in the present study by measuring rates of encroachment of papyrus from the banks into the channels, rat es of debris production from the channel fringes, quantities of debris flowing along different channel sections, as well as the dimensions, colonization, decomposition and overall spatial dynamics of debris blo ckages along the lower reaches of the major distributary channel of th e Okavango Delta. Results do not support suggestions that papyrus grow th and/or debris production cause channel blockage and abandonment. Fi rst, encroachment is inversely related to current velocity within the channel fringe, and current velocities in the fringes of those distrib utary channels that are considered to be prone to blockage and abandon ment are amongst the highest in the study area. Secondly, channel cros s-sectional area is maintained by erosion of the channel bed beneath f loating debris blockages. Thirdly, there is no evidence for the gradua l upstream development of debris blockages in the lower reaches of maj or distributary channels. Data on hydrological aspects on the lower re ach of a major distributary channel system suggest that sedimentation processes, leading to aggradation of the channel bed and to a decline in current velocity may be the cause of channel decline. This appears to be accompanied by encroachment of the channel from the margin by pa pyrus and by the development of more permanent blockages than were obs erved in the present study. This sequence of events (encroachment and blockage) is therefore considered to be a symptom and not the cause of channel decline and abandonment.