Steinbeck uses the opening chapter of his novel for many different purposes. His evocation of the beauty of nature suggests the serenity that all his readers would like to enjoy, and it contrasts with the brutal life on the ranch where wage slaves wrest profit from the ravaged soil for the benefit of the owner. George deliberately chooses to spend the night sleeping on the ground, rather than proceeding just a quarter of a mile to the ranch where they could get bunks and a meal. When Lennie asks:
"George--why ain't we goin' on to the ranch and get some supper? They got supper at the ranch."
George rolled on his side. "No reason at all for you. I like it here. Tomorra we're gonna go to work. I seen thrashin' machines on the way down. That means we'll be bucking grain bags, bustin' a gut. Tonight I'm gonna lay right here and look up. I like it."
The quiet setting gives George and Lennie their first chance to talk about their relationship and their prospects since they fled from Weed, so Steinbeck can use the chapter for a lot of exposition in dialogue form. The setting establishes and also foreshadows where they will rendevous if Lennie gets into any trouble at the ranch; and that, of course, is what happens in the last chapter. George and Lennie are still free in the first chapter and relatively untroubled. This suggests the sort of freedom and simple pleasure they dream about achieving on a permanent basis if only they can acquire a little farm of their own.
In addition to all of this, the mood evoked by Steinbeck's description of the peaceful setting subtly foreshadows George's decision to kill Lennie. George knows that helping Lennie to escape the lynch mob and then turning him over to the law, as he would have to do, would mean that Lennie would probably spend the rest of his life in confinement, possibly even solitary confinement, since he would be regarded as a homicidal maniac. The setting by the beautiful river symbolizes everything that Lennie would never be able to experience again. This deprivation of freedom to experience the beauty of the outside world is probably the worst feature of institutional incarceration.
Lennie is a lot like a wild animal himself. In Chapter 1 he talks about going off into the mountains and living in a cave. He would suffer more from confinement than many other men, and his situation would be worse because he wouldn't understand why it was happening.