The Congo River symbolizes Marlow's journey from innocence to knowledge and experience. Marlow recognizes this, saying to his listeners that traveling the river to Kurtz's domain was
the culminating point of my experience. It seemed somehow to throw a kind of light on everything about me—and into my thoughts.
The river, therefore, symbolizes Marlow's struggle to move away from the trappings of European civilization and into a different atmosphere where the veils of civilization as he knows it will be ripped away to reveal a "heart of darkness" at the center of all things—particularly at the core of the imperialist enterprise.
The journey up the river into awareness is a difficult one for Marlow, fraught with danger. Marlow describes the river as seeming
to beckon with a dishonouring flourish before the sunlit face of the land a treacherous appeal to the lurking death, to the hidden evil, to the profound darkness of its heart.
The above quote makes clear that the Congo will lead Marlow to an encounter with evil. This encounter with darkness and death is tempting to him in the same way that the tree of the knowledge of good and evil tempted Adam and Eve in the Garden of Eden: it "beckon[s]" and it has a "treacherous appeal." Marlowe is both attracted and repelled by the river, just as he will both admire and see the evil in Kurtz.
The Congo River also shrouds evil in mists and white fogs and the dense, black, tangled web of the jungle that seems impenetrable to Marlow as he travels toward Kurtz. The river, therefore, paradoxically both brings Marlowe face to face and darkness but provides a separation it. It is a conduit, not evil itself.