When there’s as decent amount of accumulation of snow or ice, especially on the street, and it rains, parts of the snow or ice on the ground melt and some doesn’t. It depends on the thickness of the snow/ice, how dirty it is, the grade of the hill it’s on, the type of ground it’s on, exposure to sunlight, etc.. What you end up with is a varied environment of hills of snow/ice, valleys, real running rivers, swamps, lakes. Essentially it’s a miniaturized, ersatz geography.
There’s a neat metaphor in all of that somewhere, but it doesn’t feel like I’m the one destined to engineer it. Anyone reading this should feel free to take it if strikes you.