Fluvial islands

There are different ways to visualize the garden of Eden. There are several reasons for that. We weren't there. It no longer exists. Gen 2 gives some basic details, but is fairly sketchy. 

It's a useful exercise to mentally reconstruct Biblical scenes. In Gen 2-3, you have a clear-cut distinction between the world inside the garden and the world outside the garden. That raises the question of natural barriers. There are different kinds of natural barriers. One possibility is that Eden was located in a narrow river valley, where steep hills separated Eden from the outside world.

In theory, water can be a natural barrier. Take a tropical island, surrounded by the ocean. Consider the fabled island paradise of Dilmun. However, the geographical markers in Gen 2 are centered on rivers. 

Mind you, some rivers are wide enough to have islands (fluvial islands or river archipelagoes). Take the Brazilian island of Marajó (situated at the mouth of the Amazon River), the size of Switzerland. Bananal Island is another example. That's upriver. 

Moreover, some fluvial islands have tidal rivers. They have rivers inside and out. 

On a related note are river deltas. Indeed, there's the Tigris-Euphrates delta. Of course, the topography has changed over the millennia. 

But just as water can be a natural barrier, absence of water can be a natural barrier. Eden was lush because its rivers provided natural irrigation. But by the same token, it might have been surrounded by desert. Expansive deserts can form impenetrable barriers for many animals. 

This might link Gen 2-3 to Gen 1:28. Why the command to subdue the natural world if the whole world was paradisiacal before the Fall or the Flood? Well, perhaps because the prelapsarian, prediluvian world wasn't paradisiacal in general. Eden was exceptional, due to its auspicious location. Indeed, God was the Edenic landscaper.