First, the river would dry up, causing the river beds to become foul-smelling swamps. Later, the branches, reeds, and rushes would start rotting. Descriptive words are used to emphasize the dire situation.
The Egyptians were dependent on the Nile for their daily existence. The river was seen as a gift from the gods, and drought was interpreted as the wrath of the gods.
5 And the waters of the sea will be dried up, and the river will be dry and parched,