Final answer:
The Nile River is the significant water body that separates Upper Egypt, which is in the south and at a higher elevation, from Lower Egypt in the north.
Step-by-step explanation:
The river that splits the upper and lower regions of Egypt is the Nile River. Ancient Greeks, like Herodotus in the fifth century B.C.E., talked about Egypt as the "gift of the Nile" due to the immense impact of the river on the region's development and sustainability. Egyptologists have determined that the first dynasty of Egypt began around 3100 B.C.E., ruling both Upper and Lower Egypt with a clear demarcation created by differences in elevation along the Nile. It is important to note that Upper Egypt is located to the south while Lower Egypt is to the north. This is because the Nile River flows from south to north, making Upper Egypt at a higher elevation relative to Lower Egypt.