What does Esperanza mean when she says to Miguel that there is a " deep river" that runs between them? Describe how their relationship changes in California and why.

But now that she was a young woman, she understood that Miguel was the housekeeper's son and she was the ranch owner's daughter and between them ran a deep river. Esperanza stood on one side and Miguel stood on the other and the river could never be crossed.

This means that Esperanza realizes that Miguel is the son of the housekeeper, and that because of her family's status, they will never be together.