Mood is achieved in literature by combining the setting of the literary piece with the tone and the theme, among other factors. These elements are put together to create a sensation in the reader that is designed to make better text- to-self connections.
The mood in the first part of paragraph 1 of the novella Of Mice and Men can be accurately described as "peaceful" and "idyllic." The images conveyed by Steinbeck are picturesque, pastoral and ideal. This is how he achieves the peaceful mood: by bringing all the beautiful elements of nature and putting them together in their most poetic form.
Hence, in the beginning, the narrative is still far removed from the chaotic life stories of the main characters. Steinbeck only focuses on the setting, which is described with a combination of bucolic illustrations and references to nature that elicit in the reader a sense of peace.
The Salinas River drops in close to the hillside bank and runs deep and green. The water is warm too, for it has slipped twinkling over the yellow sands in the sunlight.
The reader, still unfamiliar to the main story, may erroneously assume this description is setting the tone for the rest of the novella, and that, maybe, what we will read about will maintain this colorful backdrop throughout.
On one side of the river the golden foothill slopes curve up to the strong and rocky Gabilan Mountains, but on the valley side the water is lined with trees—willows fresh and green with every spring. . . and sycamores with mottled, white, recumbent limbs and branches that arch over the pool.
In reality, it does. The mention of the sycamore trees starts off and ends the narrative in chapter 1, which basically shows that, regardless of the tragedies in the lives of humans, nature does not alter and continues to envelop us.
The mood of the first chapter changes drastically once George and Lennie start interacting. Here is where we realize their story is anything but idyllic or picturesque, much less ideal.
We could just as well of rode clear to the ranch if that bastard bus driver knew what he was talkin' about. . . God damn near four miles, that's what it was! Didn't wanta stop at the ranch gate, that's what. Too God damn lazy to pull up.
Therefore, the chapter starts with an ideal view of reality, where nature is described purely and beautifully right before the human factor comes in contact with it. Once Lennie and George enter the scene, everything changes. The harsh reality of their daily existence is made evident. The mood switches, from ideal to uncomfortable. It is clear, from that point on, that what awaits the men will not be a source of beautiful things. We will find out as much as the plot moves forward.