Gbeatty's answer is correct, and terrific, as always, but I wonder if you might be looking for some physical description of the Valley itself.  The Salinas Valley was very important to Steinbeck.  He was spent about two-thirds of his life there, and was always enchanted with the fertility and beauty of the land.

In the opening sentences, Steinbeck describes the the valley this way:   "On every side it sat like a lid on the mountains and made of the great valley a closed pot."  The bottom of the valley is "broad"; the rising foothills are covered by "yellow stubble fields."  The river is flanked by "thick willow scrub" and "flamed with sharp and positive yellow leaves."  The soil is rich and black, the weather often sunny but sometimes foggy. 

Eliza is like the Valley in that she has the potential for fertility and beauty, but like the tender flowers she cultivates, it will take nurturing to bring out her to bloom, care she is unlikely to get from her husband or random passers-by.

The story is actually in the Salinas Valley. Beyond that, readers aren't given enough information to locate it much more specifically. We know the ranch is in the foothills, because we're told that--but that leaves many options. It could be either side of the Salinas river, since there are mountains on both sides. Because there's no mention of salt /sea air, I'd guess is on the east side of the river. The Western Meat Company was in Fresno, which also makes me the story's set on the east side of the river. They're within a couple of hours travel of Salinas itself, and close enough to the highway to know how the road crosses it, which puts them pretty close to highway 101...but that's about as far as we can pin it down.