edge of the water

This can refer to the surface of the water as well as the bank where the water flows to dry land. (See: rc://en/ta/man/jit/figs-synecdoche)

now the Jordan overflows all its banks throughout the time of the harvest

This is background information and it emphasizes the scale of what Yahweh is doing. (See: rc://en/ta/man/jit/writing-background)