In season 3 episode 2 of Survivorman, Les Stroud explains that red snow will make you sick. He also says that he doesn't know why it makes you sick. What's in it that makes you sick?
As Giersch and National Park representatives explain, the red pigment in the algae helps to protect its chloroplast from the sun’s radiation, allowing it to take in more heat. As the algae absorbs the sun's heat, it causes the snow around it to melt, feeding the algae with water and allowing it to continue thriving as it settles into deposits in the snow.
That being said, NPS representatives strongly advise against any temptation to taste the sweet-smelling ice, as scientists warn that watermelon snow can cause disruption to your digestive system.
Scientists, however, warn us not to eat watermelon snow because of its laxative effects.
That being said, it's usually concentrated in areas and I have never had a choice between watermelon snow and no water.