Linear dune pattern variability in the vicinity of dry valleys in the southwest Kalahari

Linear sand dunes in the southwest Kalahari dunefield have been widely observed to change pattern, size and orientation in the vicinity of river valleys, yet few explanations for this change have been put forward. This paper identifies and describes four main associations between dunes and valleys which occur in the southwest Kalahari: (i) dunes which adjoin the valley on both sides with no discernible change in pattern or orientation; (ii) dunes which adjoin the valley and change pattern; (iii) dunes which adjoin the upwind valley flank but not the downwind side; (iv) dune-free zones on both sides of the valley. The two main factors which influence the evolution of these associations are identified as the effect of the valley upon sediment supply and the effect of the valley upon wind regime. Our current state of knowledge about the potential geomorphological effects of these two variables is evaluated with reference to each association described.