A foot of snow will make it sloppy the weight will cause the water to seep through any openings/cracks in the ice. Will be a slushy mess . It does not really "hurt" the safety of the ice assuming you have a decent 4-5" to begin with. I live in lake effect snow country and deal with feet of snow regularly. Over time this can make white ice, but it takes a while. I can't find any actual good ice to fish this year, everytime the lakes cap over the snow stops the ice at an inch or 2 it is driving me nuts, but if there were enough ice the knee deep snow wouldn't stop me.