The Salinas Valley runs from King City northwest to Salinas, which is south of San Fransisco Bay. "Salinas" is Spanish for salt marsh, salt lake, or salt pan. The area is largely used for agriculture. The area is ideal for vineyards as well as crops such as broccoli, lettuce, spinach, and peppers giving the valley the nickname "Salad Bowl of the World" or "America's Salad Bowl."
Steinbeck used this area, his home town, for multiple works such as East of Eden and Of Mice and Men. George and Lennie are itinerant ranch workers. They find themselves in this area which is prime agricultural land. This adds tragic irony to their desire to one day own their own farm. The river itself is a symbol of an escape from society. It is a place of refuge for times when Lennie gets into trouble.
A few miles south of Soledad, 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 before reaching the narrow pool.
The landscape is described mostly in terms of wildlife but in a silent, peaceful setting and it is therefor a place where Lennie is away from other people and least likely to get into trouble.