Where was (is?) the Garden of Eden? (Genesis 2:10-14)

According to the text, a river came out of the Garden of Eden.  This river had four branches, the Pishon, Gihon, Tigris, and Euphrates.  We know where the latter two are - they are in present-day Iraq.  However, the first two remain a mystery.

A few things come to mind.

The Garden of Eden river seems to run backwards.  Branches flow into a river.  They do not split off from a flowing river.

To a reader of long ago, I think the inclusion of the Tigris and the Euphrates would indicate that the entire Mesopotamia region is watered by the river coming from the Garden of Eden.  We also know what happens in Genesis 3.  The flowing of the Tigris and the Euphrates did not stop with events of Genesis 3.  So, the effect of Eden to be a spring to the Mesopotamian world did not entirely stop at Genesis 3 either.

I have seen modern attempts to locate where the Tigris and Euphrates would have met and possibly where two other rivers joined it.  Although I do not have a cited source for this information, I believe there is such a location under the current day Persian Gulf.  When sea levels were lower 10,000 years ago, this location existed.

But really, I think that is beside the point.  They're still looking for a convergence of four rivers, not a backwards flow divergence.

Also, the text provides the most detail about the location of the Pishon and the Gihon.  This suggests that the author did not expect the reader to be familiar with their locations.  The 

Pishon is given the most detail and is associated with deposits of gold, onyx, and pearls.  The location of the Euphrates is understood for the reader.

The location of these four rivers, two of which are known, two of which are unknown, connects the paradise of Eden to our actual world.  This reinforces a subtle notion that emerged in Genesis 1 and we see contrasted in Genesis 2.  That is that Genesis 1 speaks of all humanity and the entire earth.  Genesis 2 focuses in on two individuals, Adam and Eve, in a particular place, Eden.  The location of these rivers bridges the gap between these two chapters.  Yet, this location is still mysterious since we do not know the exact identity of the Pishon and Gihon, nor is it reconciled how a river can flow backwards into four distinct branches.

Finally, jumping ahead to Genesis 3, I think this exact mystery is fine given that we will also see a change in the nature of animals.  If animals biologically changed and herbivores become carnivores in Genesis 3, then it is fine with me that the four rivers of Eden change as well.