Eden represents the brain, head, or consciousness itself as a whole. The Garden that was planted Eastward represents the female, creative aspect of the subconscious, i.e. abstract thoughts. "East" obviously signifies to the right, and the right brain always signifies the female aspect of the conscious or subconscious, so it matches up quite nicely.
When going from a "primitive" stage to a more aware stage of consciousness, abstract thoughts are necessary to make you start questioning your own reality. If you think about the domesticated animals, like cats and dogs, who seem to have more consciousness than wilder or more socially primitive animals, like frogs or moles, one thing that separates them is the fact that cats and dogs have dreams, or at least much more vivid ones, and also have more time to "meditate" throughout the day naturally as they don't have to worry about hunting for food or running away from anything thanks to their helpful human pals. They spend a lot of time just sitting and staring, which is basically just a form of meditation or deep thinking. As a result, they are more aware of a broader scope of reality when compared to other "inferior" animals.
In fact, even non-domesticated animals who are aware of a broader scope of reality will naturally move up the food chain and evolve faster. If you're not aware of a broad scope of reality, like a flamingo for example, you're going to get mauled out of nowhere by an animal that DOES have a broad scope of reality, such as a crocodile. As you move up the food chain, you naturally have more free time to "meditate" or deep think, which in turn aids in evolving you're hunting and defense skills. This is all just using the Lower forms of consciousness, though, and it changes drastically once the Higher forms are discovered.
So after the Garden was planted Eastward, God then put the man who he had formed. This explains the conjoining of the analytical lower male consciousness (man) with the abstract lower female consciousness (Garden) in more ways than one.
In a psychological sense, the lower feminine aspect raises thoughts and questions naturally in the subconscious, while the male aspect tries to makes sense of them with conscious critical thinking. Another way of saying this is that the feminine (subconscious) is impregnated by the masculine (conscious) (man is PLACED IN Garden), and then births the reality that a human perceives. This is the beginning of mankind and what begins to separate human from beast.
In an astrological sense, Eastward represents where the sun rises. The Garden being planted Eastward ensures that the Sun will rise from there every day. Before the sun rises, it is night time, which is another connection to feminine "lunar" aspect of subconsciousness. But since the Sun, or masculine "solar" conscious aspect, is inevitably going to rise there, it brings Light and logic for Understanding. In other words, God gave us the natural feminine ability to have basic abstract thoughts, but also gave us the masculine ability to be aware of them and analyze them rather than just ignore them like animals do. Sunrises are notoriously known throughout ancient mythology to be metaphors for having a profound thought, back in the days when Knowledge equated to Power. Think of it as God ensuring us the ability to have "mental sunrises" by strategically placing the "Garden" "Eastward" where the sun always rises.
In an evolutionary sense, this is also symbolic of the birth of any given human being. During their infancy, they are all about analyzing things and figuring life out, which is all thanks to the male aspect of consciousness. It is not until their toddler stages that they are introduced to the metaphorical garden and start using their feminine, or imaginitive, aspect of subconsciousness to further perceive the true nature of reality.
"Trees" from the "ground" in this line represent thoughts that sprout from the lower consciousness, Garden or brain. All of the thoughts we had in the Beginning were pleasant to the sight, because we had no knowledge of Good and Evil or Duality yet. Every thought was new and awesome to realize.
The Tree of Life and the Tree of Knowledge of Good and Evil are thoughts that already existed, but we weren't aware of yet because we hadn't raised our consciousness enough to know. God put them in our brains for us to discover, though. And soon enough we'll find out how we discovered them through deep meditations.
Again, there is symbology to any given human here as well. When you are a child, i.e. in the Beginning, you only have good thoughts because you have yet to understand the difference between good and evil, duality, or be controlled by the ego.
Here the author is making a reference to the spinal cord and to the four total aspects of consciousness. The "river" is the spinal cord which connects from "Eden" or brain down to the point of the base chakra and carries energy or spinal fluid to and from the brain during meditation. Consciousness is separated into four with a High and Low level, and each level consisting of both Feminine and Masculine aspects. Notice how in line 2:10 it even mentions how they became four "heads". This describes the Creation of the four forms of consciousness.
As of right now in this story, mankind is still in the lower form of consciousness, still not yet aware of Good and Evil or ego or higher forms of consciousness. Pison, the river described as having the gold that is good, is the southern most river when looking at a map of the literal interpretation of the Garden of Eden. Southernmost would signify the lower consciousness, which would go hand in hand with what I previously pointed out about the Trees or "thoughts" all being "good" in the lower forms of consciousness as well, just like the gold from this river. The Hiddekel river on a map is up North, and in the verse it says it flows East, which indicates this river represents the path to Higher Female consciousness.
This also describes the beginning stages of a child's life when he or she is just starting to realize there is more to life than what they thought. It describes the time in a child's life when these higher forms of consciousness start sprouting and branching out.