Symbolism, Imagery, Allegory
We have to admit—this book makes us want to go down the river on a raft. Possibly for the rest of our lives.
But it's not just because the river is tranquil, soothing, and the best place to chill beverages (a cooler fits perfectly in an inner-tube). It's because, after reading Siddhartha, we have a hard time not thinking of rivers as wellsprings of ancient wisdom about the fleeting passage of time. In a good way.
The river is a central symbol in Siddhartha, representing unity and the eternity of all things in the universe. At times of great transition in his life—such as when he leaves the Samanas and later when he abandons his wealth—Siddhartha returns to the river.
Eventually, as Siddhartha studies the river and comes to recognize it metaphorically for all that it represents about existence and time, he is able to attain enlightenment:
"You've heard it laugh," he said. "But you haven't heard everything. Let's listen, you'll hear more."
They listened. Softly sounded the river, singing in many voices. Siddhartha looked into the water, and images appeared to him in the moving water: his father appeared, lonely, mourning for his son; he himself appeared, lonely, he also being tied with the bondage of yearning to his distant son; his son appeared, lonely as well, the boy, greedily rushing along the burning course of his young wishes, each one heading for his goal, each one obsessed by the goal, each one suffering. The river sang with a voice of suffering, longingly it sang, longingly, it flowed towards its goal, lamentingly its voice sang.
"Do you hear?" Vasudeva's mute gaze asked. Siddhartha nodded.
"Listen better!" Vasudeva whispered. (11.12-15)
Note that the river doesn’t bestow enlightenment in and of itself—it helps direct the thoughts of someone who is ready to listen.