Can someone identify this book?

Reader Mama Bean left this comment on a post that wasn't read much:

This brought to mind a book I read in my early teens about a girl and her younger brother in some dystopian future where everything is dry, and they go searching for a waterfall on a postcard their parents left them, but when they get there, it's just a little trickle. I am hoping one of your readers knows what book I'm talking about and can refresh my memory on its title. 

I'm stumped (and I like dystopian future stories), but perhaps somewhere out there is an omnivorous reader or an astute librarian who will know...