According to you, so was the entire continent. I suppose we're expected to assume Genesis's original readers simply took that for granted?
That some geography has changed (and no one denies that some rivers dry up or change course) doesn't imply all of it has. It simply renders the text of the Bible nonsensical if it leads readers to believe the rivers flowing out of Eden were the Tigris and Euphrates they were familiar with, when it was actually closer to the Mississippi Delta.
You could have a point, but it's not as tidy as you may think.
First, no one is arguing for Florida, so just stop.
Second, the river that watered Eden is unnamed. From Eden it turned into four rivers that watered other regions. Clearly, Eden is no more, and was gone long before the time of Moses, as well as its river, the source of the other four.
The tenses employed may not as conclusive as it appears on the surface.
Here's the passage in the Septuagint:
And a river proceeds out of Edem to water the garden, thence it divides itself into four heads.
The name of the one, Phisom, this it is which encircles the whole land of Evilat, where there is gold.
And the gold of that land is good, there also is carbuncle and emerald.
And the name of the second river is Geon, this it is which encircles the whole land of Ethiopia.
And the third river is Tigris, this is that which flows forth over against the Assyrians. And the fourth river is Euphrates.
The English Standard Version
A river flowed out of Eden to water the garden, and there it divided and became four rivers.
The name of the first is the Pishon. It is the one that flowed around the whole land of Havilah, where there is gold.
And the gold of that land is good; bdellium and onyx stone are there.
The name of the second river is the Gihon. It is the one that flowed around the whole land of Cush.
And the name of the third river is the Tigris, which flows east of Assyria. And the fourth river is the Euphrates.
It may be that the present day Tigris and Euphrates are segments of their original selves. But they're flowing on the surface of hundreds and thousands of feet of layered sediments rife with marine fossils that buried the lush antediluvian vegetation that is now being pumped out as oil. Still the region that was Eden is lost.