Next time you're with a friend on top of Mount Washington looking down at the fountain at point state park, ask them how many rivers there are.  If they're completely clueless of pittsburgh's history, I could see someone saying just 2, assuming that either the mon or the allegheny river just flow into the other.  If they said 3, they're still wrong.  It's straight out of a DaVinci code or Nationaly Treasure plotline, but the fountain is actually supplied by an underground water souce.  I've always known this, but a recent article in the paper reminded me of that fact.  So the real answer would be 4 rivers would be in plain sight.

for more on the under ground "river" - http://www.clpgh.org/exhibit/neighborhoods/point/point_n77.html