In cold places (or simply in winter) instead of raining, it snows.
Snow acts exactly as water with some exceptions. For example, it accumulates in waterlike depths (1-7/7), it drowns drownable creatures, it's blocked by doors and hatches, it fallw thriugh grates it reacts to magma as water, etc.
Water melts snow, incorporating it on equal amounts (to simplify).
Main exceptions to waterlike behaviour: snow doesn't move by itself: a 7/7 of snow will remain as it is, until melted by temperature or water. However, snow's does move by pressure.
Those two characteristics make snow end up laying on a uniform 7/7 layer; as snow piled on top of snow push pressured snow to close-by free tiles. This behaviour also creates "glaciers", snow accumulating valleys, etc.
One more rule: If piled snow can't move away, once it's three levels high, it compresses to one level of ice (as current ice) and one of snow on top.
Comments? Ideas?
[Edit: I forgot to say the point of all this is to have large amounts of snow piling on valleys to then melt on summer and create or overflow the existing river. So this needs rivers to not accept endless amounts of water.]