Crossing snow layer melting underneath

She'll be right, until it isn't. No comment about snow bridges, but new snow falling on an icy surface is classic avalanche. I've heard you've got about 10min to be rescued alive from being snow buried. Your breath forms a small melt bubble that then glazes over at the snow face. You're now re-breathing the contents of a glass bowl. Manage your risk.

Thick snow covering a valley stream in summer? You're talking about avalanche debris right? It's usually pretty hard and stable (until it's not). It will have fallen a considerable way, partly melted due to the friction as it fell then set again like concrete when it stops. But then it will eventually become rotten due to summer heat and melting and the stream running underneath it. I'd look at it, kick steps in the edge of it, is it concrete like, hard to kick steps in or soft and rotten? Is it easy or impossible to push an ice axe shaft or walking pole into it? Is it a single thick concrete like mass or are there signs of cracking? I'd be pretty nervous about a 5 m high (10 m wide?) arch, I'd want it to be several metres thick of compacted snow which you couldn't push an ice axe into more than the top 10 cm or so with no evidence of cracks at all. I'd be probing ahead with my ice axe shaft and or walking pole before each step making sure it maintained a hard concrete like consistency. Probably I'd give up and go home or try hard to find a different way across if the consequences of getting it wrong were so serious. Smaller arches in hard compacted avalanche debris are generally not a problem, though I'd be watching carefully and probing with the ice axe shaft to assure myself it was good and hard all the way across.