John Steinbeck's The Grapes of Wrath is set in the 1930s in the midst of the Dust Bowl and the Great Depression. Life is hard for the Joad family before they get evicted from their land and begin their journey to California, but it is about to get much more difficult.
Noah Joad is the oldest of the Joad children, and he has always been just a little different. Noah's father helped deliver the boy before the midwife arrived, and he feels guilty that Noah's condition (whatever it might be) is his fault.
When the family finally crosses the Colorado River into California, they stop and spend the day, as they will travel through the desert at night. This is where Noah tells Tom that he will be staying here rather than going on to California, explaining, "'You know how the folks are nice to me. But they don't really care for me.'" It does not appear that Noah is staying because he is bitter or angry but because he senses that he will be better off staying here.
Of course Tom tries to convince his brother to change his mind, but Noah is determined. Noah says,
"It ain't no use. I was in that there water. An' I ain't a-gonna leave her. I'm a-gonna go now, Tom--down the river. I'll catch fish an' stuff, but I can't leave her. I can't."
This is a significant moment, as Noah is the first one in the Joad family to leave voluntarily. He might intuitively know that he is not at all suited to the rigors and hardships ahead of them in California or perhaps he is just tired of the struggle; but he certainly knows this is a place where he feels comfortable and at home, confident that he will never starve living next to a river.
Ma Joad cares only about feeding her family and ensuring that they stay together. Her father dies early in the trip and of course is not traveling with the family any more; however, Noah's leaving represents the real beginning of her family's slow deterioration because he goes willingly. He is the first, but others will also go. Noah's leaving is literal, but is also a symbolic first step to the erosion of the Joad family.