Albert Barnes 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 “streams” mean the natural branches of the Nile in Lower Egypt. The word “rivers” should more accurately be “canals”; they were very extensive, running parallel to the Nile, and communicating with it by sluices, which were opened at the rise and closed at the subsidence of the inundation. The word translated as “ponds” refers either to natural fountains or, more probably, to cisterns or tanks found in every town and village. The “pools,” literally “gathering of waters,” were the reservoirs, always large and some of enormous size, containing sufficient water to irrigate the country in the dry season.

In vessels of wood - The Nile water is kept in vessels and is purified for use by filtering, and by certain ingredients such as the paste of almonds.