Albert Barnes Commentary Psalms 78:44

"And turned their rivers into blood, And their streams, so that they could not drink." — Psalms 78:44 (ASV)

And had turned their rivers into blood - (Exodus 7:20). Properly, there was only one river in Egypt—the Nile. But there were several branches of that river at its mouth, and numerous artificial streams or canals cut from the river, to any of which the word 'river' might also be given. Compare the notes at (Isaiah 11:15).

And their floods ... - Their streams; the canals and branches of the Nile, where they usually obtained a supply of water.