STAGE, DURATION, AND TRANSMISSION LOSSES OF KUISEB RIVER FLOODS ACROSS THE NAMIB AND VEGETATION DYNAMICS AND DISTRIBUTION ACROSS FLOODPLAIN AND RARELY-FLOODED DOWNSTREAM REACHES

Flooding and vegetation across the Namib ephemeral streams have been discussed before. We calculated flood discharge associated with channel (“bankfull), top of sand bars, and floodplain of the Kuiseb River and tie vegetation distribution and responses to computed flood magnitudes, stage, and flow duration in channel and floodplain. ~100 m³ sec^-1 are the “bankfull discharge”, 300-400 and ~600 m³ sec^-1 cover most and all the floodplain, respectively. This (modelled) rating curve and the flood magnitude series indicate that only during short duration flood cover and infiltrate through the floodplain, and recharge the shallow aquifer away from the main channel. We conducted flood routing modeling, direct measurements of transmission losses in channel and floodplain, documented the stratigraphy of the channel bed and floodplain, and surveyed the perennial vegetation across the river. The limited infiltration and freshening of the shallow ground water through the floodplain is manifested by increased salinity-resistant vegetation, whereas, recharge occurs through channel bed and bank. With increasing distance from the channel, the rare infiltrating freshwater mix with the more saline water. This, in turn explains the vegetation distribution across the Kuiseb River: fresh-watered perennial vegetation near the channel and salinity resistant vegetation farther away. Post-flood germination of sensitive-to-salinity trees occur after floodplain flooding. However, they dry out within months to a few years and the more salinity-resistant vegetation remains. Only once or twice per decade Kuiseb floods reach the ocean. As a result, the above channel-vegetation dynamics, controlled by the large floods, is identified (with modification) in downstream direction, rare floods increase the arboreal area for a few years after the flood. Vegetation dynamics and distribution across and along the Kuiseb River is controlled directly by flood magnitude/stages.