Albert Barnes Commentary Exodus 8:20

"And Jehovah said unto Moses, Rise up early in the morning, and stand before Pharaoh; lo, he cometh forth to the water; and say unto him, Thus saith Jehovah, Let my people go, that they may serve me." — Exodus 8:20 (ASV)

Comes forth to the water - See the Exodus 7:15 note. It is not improbable that on this occasion Pharaoh went to the Nile with a procession in order to open the solemn festival, which was held 120 days after the first rise, at the end of October or early in November. At that time, the inundation is abating, and the first traces of vegetation are seen on the deposit of fresh soil.

The plague now announced may be regarded as connected with the atmosphere, also an object of worship.