Charles Ellicott Commentary Exodus 7:19

"And Jehovah said unto Moses, Say unto Aaron, Take thy rod, and stretch out thy hand over the waters of Egypt, over their rivers, over their streams, and over their pools, and over all their ponds of water, that they may become blood; and there shall be blood throughout all the land of Egypt, both in vessels of wood and in vessels of stone." — Exodus 7:19 (ASV)

The waters of Egypt consist of the main stream of the Nile; its branches; canals derived from it; natural lakes, pools, or ponds (either left by the inundation or formed in anticipation of it, derived by percolation from the main stream); and artificial reservoirs of various sizes in gardens, courtyards, and houses.

There is no other stream but the Nile in the whole country, and there are no natural springs, fountains, or brooks. Water may, however, at all times and in all parts of the Nile Valley, be obtained by digging wells; but, as the soil contains nitrates, the well water is highly unpalatable.

It is generally accepted that the author of Exodus shows in this verse, coupled with Exodus 7:24, a very exact knowledge of the Egyptian water system.

Vessels of wood, and vessels of stone: It was customary to store Nile water in tanks or cisterns within houses, so that it could deposit its sediment. These tanks or cisterns, found in more affluent households, were made of either wood or stone.