Complex sentence: The month of Koiak is also the fourth month of the Season of Akhet (Inundation) in Ancient Egypt, when the Nile floods historically covered the land of Egypt; they have not done so since the construction of the High Dam at Aswan.
Split into simpler sentences: The month of Koiak is also the fourth month of the Season of "Akhet" (Inundation) in Ancient Egypt, when the Nile floods historically covered the land. They have not done so since the construction of the High Dam at Aswan.