To retain some of the floodwaters for later use in irrigation during the growing season, the Egyptians built up earthen embankments to trap the muddy waters in large catch basins. Thus when Jehovah brought the first plague on Egypt, turning its water to blood, the Nile itself, the water in its canals and reedy pools, and the “impounded waters” were all converted into blood.​—Ex 7:14-25.

The Egyptian economy was completely dependent upon the Nile, which provided water for irrigation in a virtually rainless land and regularly flooded the land, leaving behind a deposit of rich silt. The food supply of the nation depended on this arrangement. Thus the turning of these waters into blood resulted in national calamity, even as the drying up of the Nile canals would forebode disaster.​—Isa 19:6; see NILE.