One good example of vivid imagery in The Grapes of Wrath is found on the first page of Chapter 17, and again on the last page of the chapter.
In the daylight [the migrant people] scuttled like bugs to the westward; and as the dark caught them, they clustered like bugs near to shelter and to water...But along the highway the cars of the migrant people crawled out like bugs...
This describes the need of the people to seek out the companionship of other families like themselves, where they were able to find a sense of camaraderie and community.
In Chapter 18, Steinbeck writes:
The water grew scarce...The sun drained the dry rocky country, and ahead were jagged broken peaks...And now they were in flight form the sun and the drought...and when the daylight came they saw the Colorado river below them...they sat in the cars looking at the lovely water flowing by, and the green reeds jerking slowly in the current.
The imagery presents the changes in the landscape. Even as the Joads and Wilson travel, while there is only water to be bought, through dried out and rocky inhospitable lands, ultimately they come to water: not just a little, but flowing water, with plants growing in it, moving about as the water travels by. That quickly, they come to what must seem to them an oasis in the wilderness, perhaps symbolic of physical and spiritual relief.
Again in Chapter 18, the importance of water is described again:
The sun sank low in the afternoon, but the heat did not seem to decrease. Tom awakened under his willow, and his mouth was parched and his body was wet with sweat, and his head was dissatisfied with his rest. He staggered to this feet and walked toward the water. He peeled off his clothes and waded in the stream. And the moment the water was about him, his thirst was gone. He lay back in the shallows and his body floated.
This moment of respite from the heat is especially vivid to the reader. It is easy to imagine the trip these people are making and the terrible heat they must battle (especially Granma, who is so sick). The burdens they carry are physical (in response to the heat and the obstacles they face daily), and emotional and mental as well.
Tom seems to be spiritually decimated, like a wasp stuck in the attic between screen and window. However, when Tom steps into the water, he is revived, body and soul. Water, the great giver of life, releases Tom from the depleting, vice-like heat so that he feels refreshed; the fact that he is floating may be symbolic of how spiritually uplifted he feels having enjoyed the water. With little to find pleasure in most of he time, he even smiles when a youngster takes a swim and scurries off seeing Tom already there.