Final answer:
The fifth river could refer to the fifth cataract of the Nile River, known since ancient times and highlighted by Herodotus in the fifth century BCE as pivotal to Egyptian civilization.
Step-by-step explanation:
The identity of the 'fifth river' isn't clearly defined in the supplied context. However, based on the historical references provided, one could infer that the fifth cataract of the Nile River might be relevant. The Nile River was pivotal to the civilizations of ancient Egypt and Nubia, with major cities and sites dating from around 3150 BCE to 30 BCE, as indicated on the map provided. The Nile up to the fifth cataract has been known since ancient times. Further emphasizing its significance, the ancient Greek historian Herodotus, in the fifth century BCE, described Egypt as 'the gift of the Nile.' This historical context is crucial for understanding the importance of river systems, such as the Nile, in human civilization and their exploration over time.