Great Lakes ice cover, winter 1970-71

"Visual aerial ice reconnaissance flights were made by Lake Survey Center and U.S. Coast Guard personnel. The flights provided data from which 59 ice charts depicting ice cover extent and characteristics were constructed. Temperatures were generally below normal during the 1970- 71 winter. Bay and harbor ice formation was first reported November 25, 1970 on Lake Superior. The time of greatest ice extent for individual Great Lakes varied from February to March. During this period maximum ice covers were estimated to be: Lake Superior, 48 percent; Lake Michigan, 27 percent; Lake Huron, 45 percent; Lake Erie, 92 percent; and Lake Ontario, near 10 percent. Ice covers began to diminish by early March, mainly on the lower lakes. By the end of April all lakes were relatively ice free; however, ships continued to experience problems with ice as late as May 8 in eastern Lake Erie and May 12 at the Soo Locks."