Final answer:
The question does not specify, but historically, plagues of locusts and frogs have been associated with the Nile during times of upheaval, and such swarms would have been obstacles for refugees.
Step-by-step explanation:
The creatures that swarmed the Nile when refugees had to get across were not explicitly mentioned. However, in historical and biblical contexts, there are records of plagues that included swarms of frogs and locusts during times of upheaval. For instance, locust swarms are well-documented for their destruction and were considered a plague in ancient times, as illustrated in the Golden Haggadah from Spain around 1320. The Nile River was crucial to the survival of ancient Egyptians, who used its resources to their fullest capacities, and would have also experienced the adversities brought forth by such plagues. During times of refugee crises, as when the Hyksos invaded the Nile delta or during the Late Bronze Age Collapse, such natural catastrophes would have compounded the difficulties faced by those trying to navigate the treacherous conditions.