Ice is lighter than water but it’s weight should displace a certain amount. If you drill a hole clean through an iceberg, the water will come up to sea level 
But what about a river or lake that’s completely iced over? Isn’t there a lot of snow and ice pressing down and fully confining the water underneath? How come I’ve never heard of water spouting through a hole or crack in the ice?
ice (without waves) forms on the normal surface. an ice sheet is large and less dense than liquid water so it stays on top.
No, because, as you pointed out, ice is less dense that water. The water displaced never rises above more than 90% of the thickness of the ice, so it could never squirt out of the hole.
I can conceive of ice coverage so complete as to form a fully sealed system with water flowing underneath under pressure. In that scenario it would be possible for water to rise above the surface of a hole or crack.
Never experienced it though. Not sure if it has ever happened in a natural system.
What would create the pressure?
Ice on a river floats, just like an iceberg at sea. If the water level in the river rises, the ice rises as well. Rising water usually makes the river wider, but then the newly exposed water near the shore just freezes and makes more ice. Similarly, if the water level in the river drops, the ice drops with it. The ice will often crack near shore and sometimes leaves large broken sheets of ice along the shoreline.
Even fairly thick ice doesn’t have the structural strength to support itself regardless of the water level.
If you cut a hole in the ice, the water level comes up to river level, just like it comes up to sea level in your iceberg.
'cause ice floats.
If the ice is thin enough, walking near a hole in the ice can cause water to well up through the hole. But if it’s that thin, I’d not be walking on it.
But then it’s not ice–it’s ice + you, which is heavier.
True enough.
Gravity?
An iced over stream is not level. There can be water flowing under the ice from higher ground to lower ground. In a scenario where there is a flaw through the ice and the stream is blocked or nearly blocked downstream it would be possible for water to flow out through the fissure.
Not sayin’ it has happened. This is a bit outside of my experience. (Streams where I live don’t freeze. Hang you are lucky if they even contain water at the moment.) Just saying that it is plausible.
Nice article I just read: http://www.unep.org/geo/geo_ice/PDF/GEO_C8_LowRes.pdf
It says the most dynamic action in a seasonal freeze is the usual spring break up. This could entail slow melting (as with lakes) but they can be violent and happen more than once in a season. It has to do with the increased resistance of a thick ice cover and the strength of melt-induced flood waters from upstream. This is what causes violent break ups, ice jams, and floodings.
So no gushing. Just a violent break up. Another article just says the water flow on a frozen river is considerably slower due to friction on the ice’s underside compared to when it’s open during the summer.
We have a lot of snow on our frozen lakes this winter.
The snow dose push the ice down and it causes slush when the water is pushed up through cracks and yes, holes drilled through the ice.
This slush makes for very difficult travel on the ice weather it be walking or snowmobiling.
More to your question, when a hole is drilled when there is a lot of weight from snow water comes out of the hole and in a very short time it can become 4-5 inches deep close to the drilled hole.
When driving my pickup out onto a lake to ice fish I will not park the truck close to the fishing spot just so that the water doesn’t come up causing slush.
On the flip side,
The lake my hunting shack is on has a river running through it and I rarely fish this lake in the winter. One time I did and because of recent knee surgery I drove my truck out on the ice to set up for the grandchildren. They were already out on the ice and when I drove up close to them they ran from the truck. They had been on the lakes like this for years and I thought it was just a game they were playing until I started drilling holes only to find there was very little water under the ice. Now to qualify this the lake is a resovoir (dam holding back the river) and depth is six feet. As I then drove off the lake I had my window open and the cracking of the ice because it wasn’t supported as the river had very little inflow was unbelievably loud and that is what the grand kids were running from!!
There was a very bad winter kill (of fish) that winter.
I will not be surprised to see many lakes with winter kills when the lakes open this spring OR will that be this summer??? :dubious:
I’ve seen what I think is this phenomenon in frozen mountain streams. The surface freezes and pens the water in so you’ve got basically a tiny little artesian spring system with water bubbling out of little holes in the ice. I’m guessing for that to work, you need a really fast freeze to get enough of the stream frozen in one solid chunk to create a significant elevation difference.
May not be common on Earth, but on other objects in space you can get cryovulcanism that can involve violent eruptions of water through ice.
I use to punch hole in my mom’s pool when the ice started to get really thick to relieve pressure. As the ice freezes it expands and the new ice is also on the bottom of the slab. There was a tight enough seal around the edge to prevent the water from seeping up around it.
So I’d take a pick axe and punch a hole. It could get surprisingly thick given the climate (above ground pool). And then some water would bubble up for a while.
Without such a great seal, this wouldn’t happen. If the weather got warm at all, the edge would thaw, the slab would start to float free and it would be fine for a while.
That is so. cool.