But my first thoughts are of the Matanuska river, and how it's gray from glacier silt. (Though if you've ever felt glacier silt, that stuff is so soft.) Then I'm thinking of how many people I know that when they were kids their family drove up to the Mat-su valley and decided to go hiking on the glacier randomly, while wearing short, t-shirts, and sandals. (Almost everyone I know has a family memeber that's lost a sandal to that damn glacier... when it melts there are going to be a hell of a lot of well-preserved kids shoes floating down the river.)
Anyway, to bring this back to Gravitation. My friend works for a Japanese tourist company, and they were in charge of plans for a group that was up here doing a commercial for drink called Pocari Sweat on the glacier. (I saw the commercial, I don't know what was going on, but it cracked me up.) And this gets back to Gravitation because in episode 11 Ryuichi is dressed up as a pacori sweat vending machine. ^_^ (Hope that link works...)