Recent Grain-Size Coarsening of Floodplain Deposits and Forest Decline along the Kuiseb River, Namib Desert, Namibia

Abstract

We investigated the flood plain deposits of the middle reach of the Kuiseb River in order to reveal the recent fluvial environment changes and forest decline. For the conservation of watershed environments, it is important to examine the relationship between environmental and hydrological changes. Fluvial deposits are useful for this as they record the past environmental changes in a catchment. Grain size coarsening was seen in the upper flood plain deposits in many places. The cause of the deposit coarsening was considered to be a relative increase in the supply of coarse material. The frequency of floods seems to have been decreased by the construction of many dams in the upper stream area, but sand dunes continue to advance on the river bed, increasing the relative supply rate of coarse material (dune sand). Notable forest declines were observed at the places where marked deposition of coarse sand had occurred, as such coarse deposits cannot retain water and are desiccated rap- idly. The subsequent severe dry conditions at the roots induce tree death.