Polynyas are large areas of open water surrounded by ice. The water is kept open by wind, warm weather, and/or by currents in the water. The word is borrowed from Russian and is pronounced pă' lǐn yă. They are often a significant hazard on early ice. Polynyas are typically 1000 feet to miles in size. They are a bit less of a hazard than smaller holes as long as they are open because they are so big they are generally easier to see and avoid.
On lakes, were currents are usually a minor consideration, they appear to be associated with deeper water that takes longer to get fully cooled off. Our experience so far is that they do not occur in the same areas every year which suggests a more complex reason for existence than just deep areas. Once formed they are so big that wind and waves impact their evolution more than with the little guys.
When they do freeze they can do so fitfully with lots of new ice holes and variable ice thickness. The ice on a newly frozen Polynya needs to be checked out especially carefully and fully if you venture out on it.
The area this polynya covered is the widest-deepest part of the Inland Sea but most years the whole thing is in by early-mid January.