Overnight implies to me that the change is on the order of "the river was there when I went to bed last night, and it was gone when I got up this morning."  The only mitigating factor is that the headline writer is tasked with the job of coming up with a title that will encourage people to read the article.  

(Though, in geological terms, a river disappearing over the course of a year is kind of like happening in the blink of an eye.)