The reference to the River Jordan comes at the very end of this book, when Curzon and Isabel flee and row across the river to New Jersey and freedom. It is only when they reach the other side that Isabel checks Curzon is awake. When he asks her where they are, she replies:
I think we just crossed the River Jordan.
This is a Biblical allusion to the journey of the Hebrews out of Egypt and to the Promised Land. The last section of their journey was crossing the River Jordan. Isabel therefore uses this allusion to refer to the way that, having crossed the river and reached New Jersey, which is a state where slaves are free, they have themselves metaphorically reached freedom. Their daring escape attempt has been successful, and they are able to now look for Ruth and hopefully live free lives.