What caused the river to stink?
Oznayim la'Torah #1: The dead fish, as implied by the wording of the Pasuk. 1
Oznayim la'Torah #2: The water stank because it turned into blood 2
Why does the Torah say that the Egyptians could not drink the river water due to the stench? One cannot drink blood?
B'chor Shor (cited in Hadar Zekenim and Riva, Pasuk 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 (on Pasuk 18) and Riva (on Pasuk 17, 2): The water merely looked like blood. 1 If not for the stench, they would have been able to drink it.