When the US Army Corps of Engineers measured Lake Michigan and Lake Huron water levels at the end of December, they discovered that the depth had receded to 576.15 feet above sea level — breaking the record for the previous all-time-low of 576.2 feet set in 1964. That’s not a record to celebrate. Numbers are complex and difficult to comprehend. But beaches along Sleeping Bear Bay offered more clarity, and sobering clarity at that. The shoreline has receded substantially from previous years, leaving behind a hard, rocky surface in places, and prompting widespread alarm.