The Jordan's waters backed up in a heap as far upstream as which city?

Answer: Adam

The water backed up as far as Adam, a city near Zarethan, while the flow to the Salt Sea was cut off; Israel crossed opposite Jericho.

“the flowing water stood still. It backed up as far upstream as Adam, a city in the area of Zarethan, while the water flowing toward the Sea of the Arabah (the Salt Sea) was completely cut off. So the people crossed over opposite Jericho.”— Joshua 3:16 (BSB)