The simple answer is that the Flood myth was a late addition to the source documents underlying the primeval history of Genesis. That is why the Pentateuch also presents the descendents of Cain and the Nephilim as alive and well when the Israelites entered the Promsied Land (cf. Numbers 13:33, 24:21-22). It also explains a myriad of weird things in Genesis, such as how Nimrod could have been the first gibbor "mighty one" on the earth when the Nephilim preceded him, why the birth notice of 5:29 is rendered nonsensical by the Flood, why Noah is suddenly a farmer after the Flood, and why his children after the Flood are characterized almost as minors when they were married men before the Flood. I go into the evidence in the detail in section 2 of the following post:
http://www.jehovahs-witness.com/10/68224/1.ashx
As for the source of the Tigris and Euphrates being the location of Eden, this is a theme that runs all throughout Canaanite mythology. The creator god El has his adobe in a cosmic mountain at the meeting-place of the heavenly and earthly deeps and at the source of the great rivers. Thus, in the Baal Epic, the god Kothar rushed "to El at the springs of the Double-Rivers, amidst the channels of the Double-Deeps. He comes to the mountain of El and enters the tent of the King, the Father of Years" (KTU 1.2 III 4-5). In another myth's El's mountain abode is said to lie "at the springs of the Two Rivers, at the meeting place of the Double-Deeps" (KTU 1.100 R 1-4). The same concept is in the OT. According to Psalm 42:4-7, the writer is on his way "to the wonderful Tent, to the House of God," to Mount Misar where "deep is calling to deep as your cataracts roar". Yahweh declares that he "builds his palace on the waters above" and "sits enthroned on the Flood" (Psalm 29:10; 104:2-3), and Ezekiel 28:2 has the Phoenician king declare: "I am El ('l), in the dwelling of the gods ('lhym); I dwell in the midst of the seas (ymym)," and then v. 13-14 goes on to locate the Garden of Eden on "the holy mountain of God," or rather, "the gods" (hr qds 'lhym). Similarly, in the Gilgamesh Epic (11.194-96), the abode of the gods lies "at the mouth of the rivers" (ina pi narati), and an Akkadian seal from Mari similarly depicts "a god of the type of El enthroned, between the springs of two streams, on a mountain. He is flanked by two vegetarian goddesses who grow out from the waters" (O. Keel, "Ancient Seals and the Bible," JAOS 1986:309). Moreover, the Canaanite snake-bite spell in KTU 1.100 R 1-6, V 60-75 describes the serpentine god Horon going to "the Tigris abounding in the rain" in the "east," where he searched through the "Tree of Death," "the tamarisk," and "the fruit cluster" to find the antidote of the snake venom. See my post on the mythology behind the Garden of Eden story:
http://www.jehovahs-witness.com/10/73244/1.ashx