Sentence:  Although rare, it takes an unearthly amount of rain to fill the Tsauchab River and penetrate the dune area, bringing water into the <WSD>vlei</WSD>. During these rare times, the <WSD>vlei</WSD> takes on a life of its own, much different than in the absence of water... 
Meaning of the word marked with <WSD> tags:  A shallow wetlands or minor lake, generally a seasonal one, or the lowland where such a wetlands appears seasonally.