There is a part of NE Brazil that is characterized by large and scenic coastal dune fields. Here is an image of one particular area that shows that and supports the contention that dunes rarely make it across rivers...except in extremely arid or extremely sand-rich environments. Click the link and explore this area, you may be amazed.
In particular, check out the panaramio layer in Google Earth for some spectacular shots. This is a relatively remote part of Brazil that verges on semi-arid depending on the moods of the ITCZ. I have been to the city of Fortaleza before and was shocked to see that many neighborhoods there are built on huge coastal dune complexes.