In Chapter 1 Of Mice and Men, the Salinas River Valley is a Garden of Eden, pristine, green, idyllic for sleeping under the stars. It is a place of freedom and natural wonder. There, the American Dream seems possible. George and Lennie are lords of creation.
However, it also a place for animals. Lennie threatens to run off and live in a cave like a bear. He laps the water up with his huge paws. And, in the end, he will be put down on its river bank like a horse. In short, the river valley is the secret world only George and Lennie know about, but ultimately unlivable, like the American dream. It is a fantasy, a place only to camp, a future scene of Lennie's mercy killing.
The bunkhouse, however, is a place where men compete over things: women, jobs, bragging rights. When Candy shows George and Lennie around, they find the place crawling with bugs (lice). It's infested, unnatural. It will also be a place of violence, where Lenny crushes Curley's hand and where Carlson takes Candy's dog to be shot.
Notice: only one location, the bunkhouse, is meant for human dwelling, and its accommodations are awful. The animals in the barn live better. The rest of the locations in the novel are more or less for animals (barn, river valley). So, both the river valley and the bunkhouse support the theme of animalistic violence and the impossibility of the American dream.
To me, one of the themes of the book is that the "civilized" world is a harsh place. In this world, the dreams of people like George and Lennie die because the world crushes them. I think that you can see this theme being supported by the differences between the way the author describes the river and the way he describes the bunkhouse.
The river is described in a really beautiful way. It seems so peacful and pretty. By contrast, the bunkhouse is an unpleasant place with many unpleasant people. By describing the river so beautifully, the author is underlining the idea that this is where dreams and good things are possible and "civilization" is where they are not.