How can there be a river in the sky?  
Let us go a little deep into this. What’s a river?  
 “A very large mass of water usually flowing from land to the sea”.
Can there be a river in the sea?                                                                             Yes !  There are fast flowing streams of water inside the sea. A very good example is the Gulf Stream.   

Fig Atmospheric river.     (Courtsey NOAA).

In a similar way, air currents flow highly moisture laden, in the form of droplets and
vapour. These originate in the equatorial region and speed towards mid latitudes.

They are several  hundred k.m. wide and extend for thousands of kilo metres. Many of them bring beneficial rain to the western parts of America and Europe. It has been shown that a typical atmospheric river can convey more water than the river Amazon.  A few atmospheric rivers are streaming in the sky at any time. Christening of this phenomenon was done by Reginald Newell and Yong Zhu of MIT in 1990.
 
The identification of these water transporting streams has been possible due to the use of 'microwave satellite imaging' which superseded 'infra red imaging'.  If such a river gets obstructed by a weather phenomenon or a mountain, an awful lot of rain pours down in a small area within a very short time. Fortunately this happens mostly above the sea, but when it occurs overland severe flooding and landslides could cause loss of lives and property. One such river produced more than 40 inches of rainfall in the mountains of southern California in four days in January 2005 which took 10 lives.