This requires using the straws that come with specific blunt brands, namely Bluntwraps. Any straw will do really. When the blunt is small enough to be placed in the straw, a river shotty is performed just the same way a regular shotty is. The main difference is that when the smoke is coming out of the straw it comes out much finer, thicker, and reliably than the end of a blunt. The term 'river' is there because it resembles a river of smoke rushing into the mouth of the person receiving the shotty.
Highly suggestible for those who have never tried it.
Jesus christ...Marilyn just gave me the sickest river shotty. That went straight to my head! God damn. Couldn't stop coughing though