Vegetation Blockages and their Influence on the Channel Flow Dynamics in the Okavango River Alluvial Fan, Botswana

Abstract

The vegetation blockages in the Okavango Delta tributaries have been a concern among the local
communities, water managers and in tourism industry. The objective of this study is to update historical
vegetation blockages reported in 1970s in the Okavango channels and to assess the blockage influences
on the channel flow dynamics. The blockages were determined by boat in 2002 in three river systems
namely Ikoga-Thaoge, Nqoga-Maunachira and Mboroga-Gomoti Rivers and further assessed at four-tofive-
year intervals until 2018. The collected coordinates using GARMIN GPSMAP 62S at the start and end
points of the blockages were added to the Okavango Delta feature dataset to ArcMap GIS 10.2 software
to map and measure the blockage lengths in km. ‘Papyrus’ is the dominant species blocking the channels
while the ‘hippo grass’ is a fringe vegetation. Five types of blockages were categorised based on their
growth and spread in different flood regimes. The hydrological data analysis showed that the ‘Intermittent
rooted emergent papyrus’ and ‘Emergent papyrus rafts’ in Thaoge River held large volumes of water in the
upstream Qaaxhwa and Weboro Lagoons from 2005 onwards causing some flow diversions to Boro River.
The ‘Surface blockage debris’ in the origin of Maunachira River did not have significant influence on its
downstream flow because of added flows via Khiandiandavhu channel. The Bokoro and Dxerega Lagoons
on the blocked Maunachira were reduced in their water surface area by 28.7% and 45.2% respectively
between 1984 and 2011. The hippo grass in Mboroga and Gomoti Rivers do not have much impact on
channel flow. It is recommended that efficient management of blockages through the engagement of
local communities and safari operators in the areas of concern would improve the flow distribution for
community resource use and tourism development.