BP338 Allen is correct, ice bridging can occur when water levels drop after the body of water has frozen. Asked a gentleman in Leadville Colorado about ice fishing the mountain lakes. His response was no way. The lakes are used to supply the city of Denver drinking water. He told me if they pull water from a given lake the water level can be up to 20 feet below the ice. He said it fails suddenly. If you are caught on it you are dead. These lakes are smaller and ice can be much thicker in proportion to other bodies of water. Second hand information, but I would not like to be the one to test it out.