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.
Animation by Jesse Allen, NASA GSFC, based on data from the Shuttle Radar Topography Mission and Landsat 7.