Because "tributaries" are streams or rivers that flow into larger streams and rivers.
Unless I misunderstand, "tributes" is the proper word, here.
Remember Mike Camerotta, Ladder/engine 11&28. He was twenty years old, the youngest member of FDNY to lose his life so that others may live.
I see...sorry for the confusion. "Tributaries" is my regular blog, and I chose it because of the definition you gave (treams or rivers that flow into larger streams and rivers) - the choice was deliberate. Have had the blog for well over a year now...I can see where you might have misunderstood though. Thanks :D