What does it mean?

When the witch came back from the river, she saw the girls running away. Then the witch said, “Let there be a wide river with a big crocodile in it in front of the girls!”

I have a little trouble about this sentence. I feel a little strange to the phrase “Let there be”. Was there the river in front of the girls originally? Or, did the witch put the river in front of the girls using her magic?

This means that the witch put the river in front of the girls by using magic. She conjured a river by casting a spell.