One of the reasons I am so reluctant to call Erg Chebbi a desert is because on my previous seven visits to Merzouga it's been raining four times and hail storm on another. This time the rain had ceased the previous day but strong winds were blowing sand everywhere.
The water table is only a couple of metres down in many places. Normally the Ziz and Rheris rivers don't get further than Rissani, but the rainfall has been such that the seasonal lake, Dayat Srij, was filling up whilst I was parked there, see video at http://www.youtube.com/watch?v=zfH1zB1IDso
One of the fords I crossed on the way, http://www.youtube.com/watch?v=p7kfqhcf7EY