The Pishon (Pison) River is recorded in Genesis 2:11, as one of
the four rivers emanating from the Garden of Eden: "The name of the
first is Pison: that is it which compasseth the whole land of
Havilah, where there is gold. The second River was Gihon (Nile)
that ran through Ethiopia. The third was Hiddekel (Tigris) which
ran towards Assyria. The fourth was the Euphrates, as well known
today.
The Pishon River is "most probably identified with the Karun
River which comes down from the mountains of Media and formerly
emptied into the Persian Gulf." International Standard Bible
Encyclopedia.
Whereas we are familiar with the Nile, Tigris and Euphrated
Rivers today, there is no reason to consider the Pishon to have not
existed. All these rivers are undoubtedly changed somewhat from the
days of Eden, due to the Genesis Flood.