The Kuiseb River in Namibia is bordered on one side by the tallest sand dunes
in the world, and on the other by barren rock. The red sand dunes south of the river
reach heights over 150 meters. The prevailing winds blow the dunes northward, but
their movement is blocked by the river. In the process, so much sand and
silt is deposited in the Kuiseb that it only reaches the sea while
it is in flood.