This plague against Egypt-the turning of water into blood-covered all of the water sources of the land. Included were the Nile River, other flowing streams, irrigation canals, and ponds that held water for the watering of animals and for irrigation purposes.
In the dry climate of Egypt, irrigation of crops was essential. Large holding ponds were constructed to catch water from the Nile during its annual flooding stage. This water was released through irrigation canals during the dry months.
This plague on the Egyptian water supply was a strike against the livelihood of the land and its people.
EXODUS: Say unto Aaron, Take thy rod [staff, NIV], and stretch out think hand upon the waters of Egypt, upon their streams, upon their rivers [canals, NIV], and upon their ponds, and upon all their pools [reservoirs, NIV] of water, that they may become blood,