OK, the correct answer is “I don’t know.” (But we did come across an idyllic valley during our travels in Tasmania once…) Genesis doesn’t tell us too much, and although we know of two of the rivers identified (the Tigris and the Euphrates), there’s no consensus on precisely what the other two rivers refer to.
But I want to focus on the first part of Gen 2:8, which reads:
ויטע יהוה אלהים גן בעדן מקדם
Yhwh God planted a garden in Eden, to the east.
In particular, the final phrase usually translated “in the east” or something like that, providing some sort of apparent geographical reference to the location of the garden.
The difficulty here is that although the term מקדם (miqqedem) usually means “on the east side,” it can also mean “formerly, in ancient times, way back then” or something like that. Now it is immediately apparent that the latter meaning could fit quite nicely in the context. Indeed, Westermann notes a problem for the spatial reading of the term in Gen 3:24, but is not bothered by this since he assigns the two passages to different sources and thus attributes the difficulty to sloppy editing.
Wenham, on the other hand, discounts the temporal reading on the basis of context and Gen 11:2. But these are hardly compelling, and I think Gen 11:2 actually counts against Wenham’s case. Let me explain. First, regarding the immediate context, it is clear that Gen 2 is dealing with events in the distant past, so a temporal reading of מקדם is entirely appropriate. Second, throughout Gen 3–11 there is a consistent movement to the east (cf. Gen 3:24; 4:16; 11:2) which implies that the point of origin could not be as far east as the final point at which we arrive in Gen 11, Babylon. The implication is thus that Eden lay to the west of Babylon. The further implication is that there is a symbolic reversal between the journey eastward away from Eden in Gen 3-11 and then Abram’s about face at the instigation of God as he sets off toward the promised land in the west.
Finally, Wenham notes that a number of ancient manuscripts and versions also understood מקדם temporally rather than spatially in Gen 2:8.
So, in light of these considerations, I think that at the very least Gen 2:8 is deliberately ambiguous and both spatial and temporal significance could be assigned to the phrase. But perhaps, in light of the later clues in the narrative, the temporal reading is more fitting than the spatial.1
1. It is worth noting also that there are examples where the ambiguity inherent in the term קדם seems to play a part in the meaning of the text. I think immediately of Job 1:3 and the general feeling that Job is set in patriarchal times. There is a sense, I think, that what lies in the east in such texts is also that which derives from ancient times.