Jamieson, Fausset and Brown on Isaiah 19:5

5. the sea--the Nile. Physical calamities, it is observed in history, often accompany political convulsions (Eze 30:12). The Nile shall "fail" to rise to its wonted height, the result of which will be barrenness and famine. Its "waters" at the time of the overflow resemble "a sea" [Pliny, Natural History, 85.11]; and it is still called El-Bahr," "the sea," by the Egyptians (Is 18:2; Jr 51:36). A public record is kept at Cairo of the daily rise of the water at the proper time of overflow, namely, August: if it rises to a less height than twelve cubits, it will not overflow the land, and famine must be the result. So, also, when it rises higher than sixteen; for the waters are not drained off in time sufficient to sow the seed.