Why does it say that they could not drink the river water due to the stench? One cannot drink blood!
Bechor Shor (cited in Hadar Zekenim and Riva, verse 17): It turned to blood only temporarily. When it returned to water, they could not drink it only due to the stench.
Da'as Zekenim (verse 18), Riva (verse 17, 2): The water merely looked like blood. If not for the stench, they would be able to drink it.