What would cause a hole to appear in a lake’s sand bottom after an ice out?
That would be an ice scour. During the winter, the ice penetrated down into the lake bottom; then, when the ice went out in the spring, it pulled a chunk of the bottom with it. Eventually, the hole will fill in. It’s hard to know how long this will take, though, because it depends on a few things, including the depth and size of the lake, the material at the bottom of the lake (such as vegetation and fine sand), and the strength of the lake’s waves. An ice scour is more common after a very cold winter, and on smaller lakes, when all the surface water freezes and the ice gets very thick near the shoreline.