Steinbeck uses the Salinas River to introduce the concept of man and nature in the beginning of the story.
When the story begins, we are provided with an idyllic description of a man-less nature. Steinbeck creates a picture using detailed pleasant imagery.
The water is warm too, for it has slipped twinkling over the yellow sands in the sunlight before reaching the narrow pool. (ch 1)
Other descriptions used are “golden foothill slopes,” and “trees- willows fresh and green with every spring.” These depictions are of nature doing what nature does, without interruption or imposition from man.
Onto this beautiful scene come two men who are first introduced anonymously as animals might see them. This produces the idea that the men are interrupting nature, or an impediment to nature. They have to coexist with it. This concept is further extended with the idea of them buying a farm to tend rabbits. They long to be back to nature.
The theme of nature taking its course is also foreshadowed here. We are shown George and Lennie as small pieces of a large, natural world. They are pawns of nature. They cannot escape its course.