Look at the way Steinbeck describes Lennie. He uses animal imagery and simple diction to fit with Lennie's character. After walking into the clearing, Lennie's first action is very animal-like. He falls to his knees and slurps water from the river, just as a horse might, or a dog drinking water from a bowl. George comments:
"You'd drink out of a gutter if you was thirsty."
Here we have the image of a man who is not intelligent enough to check if the water is fresh, but who also drinks in a very animal-like fashion. Lennie's mental retardation comes across clearly, as he is presented as almost less than human.