Montag escapes the police and the second Mechanical Hound by getting into a river and allowing it to bring him out of the city. The river serves as a counterpoint to the earlier comparison of the highway to a river, one rigid and sterile, the other living and fluid:
...the alley opened out on to a wide empty thoroughfare ten lanes wide. It seemed like a boatless river frozen there in the raw light of the high white arc-lamps; you could drown trying to cross it, he felt...
(Bradbury, Fahrenheit 451, Google Books)
The river also acts to remove his smell, allowing him to slip the tracking of the Mechanical Hound. While floating, Montag reflects on the simplicity of taking time to just think, instead of being bombarded by the technology and entertainment of the city; the river is strange to him, and he realizes that it is a real thing compared to the meaningless objects and opinions held in the city. He would have been caught if he had tried to escape on the "concrete river" of the highway, but the constant flow of the river allows him to escape, thwarting the government's intentions of making an example out of him.