What does Montag's swim in the river symbolize in Fahrenheit 451?

During the novel Fahrenheit 451, Montag has to swim in the river to escape the robotic hounds that are chasing him for having books. He flees his home in the city and eventually comes upon a community of book readers and transcribers trying to preserve civilization. Between his departure and the arrival at the community of book enthusiasts, he swims through a river.

This dip represents a baptism or sorts—his life is changing, as is his identity. He has abandoned his old life—the one that is full of technology and brainwashing—and is turning to a life separate from society but full of knowledge. His swim in the river washes off his scent to save him from the hounds and, in a way, symbolically washes off his old identity.

What role did the river play in Montag's escape in Fahrenheit 451?

The river plays a significant role in Montag's escape from the dystopian society. In Part Three, Montag ends up killing Captain Beatty and narrowly escapes the Mechanical Hound. After being stabbed in the leg by the Mechanical Hound, Montag manages to travel to Faber's home, where he recovers and seeks his advice. Faber tells Montag to travel down the river and follow the abandoned train tracks until he meets up with a group of traveling intellectuals. Montag follows Faber's advice and runs through the city towards the river while the policemen and a new Mechanical Hound chase him. Montag narrowly avoids the authorities by running into the river, where he floats several hundred yards downstream as the Mechanical Hound stops chasing him. Bradbury writes,

A storm of light fell upon the river and Montag dived under the great illumination as if the sun had broken the clouds. He felt the river pull him further on its way, into darkness. Then the lights switched back to the land, the helicopters swerved over the city again, as if they had picked up another trail. They were gone. The Hound was gone. Now there was only the cold river and Montag floating in a sudden peacefulness, away from the city and the lights and the chase, away from everything. (65)

The water symbolically represents Montag's renewal and rebirth as he finally experiences the tranquil natural world for the first time in his life. Montag ends up floating towards a riverbank, where he begins to walk through the wilderness in the direction of the traveling intellectuals. Overall, the river serves as Montag's vehicle of escape and provides him with an opportunity to experience a rebirth as an independent intellectual.