Sforno on Exodus 7:18/ספורנו על שמות ז׳:י״ח

1והדגה אשר ביאור תמות, the river will not retain the appearance of a river, part of it has been turned into blood; but the entire river bed will be filled with blood so that the fish have no habitat in which to survive.

2ונלאו מצרים, digging around the river bed trying to find water. This referred to what the Torah described in verse 24 ויחפרו כל מצרים סביבות היאור, “all the Egyptians dug all around the river.”