Water is necessary for life. So for a vibrant garden to flourish, plenty of water was needed. A single river (unnamed) went out of Eden to provide water for the plant and animal life there.
And a river went out of Eden to water the garden; and from thence it was parted, and became into four heads. 11 The name of the first is Pison: that is it which compasseth the whole land of Havilah, where there is gold; 12 And the gold of that land is good: there is bdellium and the onyx stone. 13 And the name of the second river is Gihon: the same is it that compasseth the whole land of Ethiopia. 14 And the name of the third river is Hiddekel: that is it which goeth toward the east of Assyria. And the fourth river is Euphrates. (Gen. 2:10-14)
This river split into four rivers, which went out to a larger land area. There are names given to these rivers: Pison, Gihon, Hiddekel, and Euphrates. Modern Bible teachers and scholars have tried to identify these with rivers seen today to try and locate the area of Garden of Eden. Pison has had several suggested identities, but in general, it is believed to be a now dry riverbed stretching across the Arabian Peninsula. Gihon is considered to be another name for the Nile. Hiddekel is associated with the Tigris River and the Euphrates River is well-known. There is only one problem with this–these rivers do not come together at any point. While it is possible that at one point they did, there is a better explanation for this: the Flood of Noah.
In Genesis chapters 6 through 8, we learn that God flooded the Earth. This massive event would have reshaped the landscape of the world. Rivers would have been reformed after the flood waters receded. Just like people have often used familiar place names when discovering new territory, Noah’s family and descendants would have used familiar names to name the rivers and other locations after the Flood. This means that the modern location of the Euphrates River is not necessarily where it was before the Great Flood. The same is true for the other rivers. This means that identifying the location of the Garden of Eden is folly.
The same is true for identifying the land of Havilah, where this passage mentions that gold, bdellium, and onyx are plentiful and good. Furthermore, the locations of Ethiopia and Assyria very likely are not the same as they were at that time. These names were reused after the Flood in a similar fashion as English settlers to the U.S. used names like York, Peterborough, Manchester, and Plymouth to designate cities.
So what is the point of listing these if we cannot identify their locations today? I think God was simply stating that His intention to fill the Earth with people was promoted by His gracious provision of water, food, and raw materials. He gave these early humans exactly what they needed to prosper in the world He had created.
For more information, please reference The Genesis Record by Henry Morris and the The Genesis Flood by John Whitcomb and Henry Morris.