Prosopis species have been introduced into Namibia in the early 20th Century, and have since invaded especially the riverine ecosystems. Within the frame of the project "A Water Secure Future for Southern Africa" a baseline study to the Prosopis infestation near Gibeon was undertaken to determine the effect of the infestation on the ecosystem, and to propose possible ways to combat this infestation. For this paper a baseline floristic survey of the invested areas is presented, using the Braun-Blanquet survey method. In addition the population density was determined using a belt transect method over 4 x 100 m. Mapping using aerial photographs from 2010 with a ground resolution of 0.5 m was done to determine the spatial extent of the identified habitat types (commonage, floodplains, river bed, and riparian thickets). Three plant communities were identified: Acacia nebrownii shrubland within the upper slope of the commonage, Tetragonia schenkii shrublands on the floodplains, and Tamarix usneoides woodland along the river banks and in the fountain area. The latter can be subdivided into three units based on structure and habitat. The population of Prosopis at Gibeon is strongly regenerating with the smaller trees and shrubs (<3 m) dominating the stands. The river bank habitat (riparian thickets) had the highest density of over 2,500 Prosopis plants/ha followed by the fountain habitat with over 900 Prosopis plants/ha. Water use by the plants was estimated for the individual habitats based on the measured densities and size class distribution, and the total water use by Prosopis for the study area is estimated at 1.2 M m3 per annum for the study area. Prosopis plants have largely replaced the natural vegetation along the Fish River, with only remnants of the original vegetation remaining. This invasion poses a major threat to the ecosystem and its functioning. The water used by the alien vegetation is considerably higher than natural vegetation would have used, and it is estimated that roughly 18% of the potential influx of the new Neckartal Dam in the lower Fish River is lost due to the Prosopis infestation.

Keywords: Alien invasive species, ecohydrology, Nama-Karoo, riparian thickets.