Some scientists have speculated that the Nile turning red in Egypt for the Book of Exodus was by a huge amount of a red bacteria, which produced a foul smell as many bacteria often do. I forgot the species of it. These bacteria sucked up the oxygen, killing the fish. To an ancient, without a microscope, it could indeed appear to be the Nile turning to blood.

Remember, God can, and does, use natural phenomena, properly timed, as miracles.