Ice Coverage on the Laurentian Great Lakes Currently (April 2014), the fraction of the Great Lakes covered by ice is more than ten times the median amount for this time of year, the median amount being indicated by the green line in the below graph and representing the 1980-2010 time period.  The source of the graph is the Canadian Ice Service.  



(in the graph, the x-axis is year, 1980-2014; the y-axis is percent ice cover 0-40%; Link to graph)

While the data in the graph are only back to 1980, looking at data from the NOAA Atlas of Great Lakes Ice Cover it can be seen that the amount of ice for this time of year is unprecedented back to 1973, the earliest year in the atlas.  

My question is, how far back would one have to go to find a year with an equal or greater amount of ice coverage on the Great Lakes at this time of the year?  Obviously the Great Lakes were glaciated thousands of years ago :), but is there any historical record of a similar ice coverage event?

update 4/29/2014:



(x-axis is year, 1980-2014; the y-axis is percent ice cover 0-30% Link to graph)

As we close out April, the amount of ice remaining on the Great Lakes is more than 4 times greater than any other year in the 1980-2014 time period, and about 50 times the median value for this time of year.  

Vast areas of Lake Superior which were considered by Canadian Ice Service as having a zero-probablity of ice-cover at this time of year are in fact covered in very thick ice:



update 6/2/14

A small amount of ice still remains on Lake Superior.  This is the first time in the satellite era (1973-) that ice has been present in June.
April and later ice cover is primarily on Lake Superior, since this lake is in the coldest region and also because it is the largest lake.  

Raymond Assel analyzed the Great Lakes ice coverage for the winters of 1897-1982 in AN ICE-COVER CLIMATOLOGY FOR LAKE ERIE AND LAKE SUPERIOR FOR THE WINTER SEASONS 1897-1898 to 1982-1983 INTERNATIONAL JOURNAL OF CLIMATOLOGY, VOL. 10,731-748 (1990).

According to table IV of Assel, in the winter of 1911-12 Lake Superior had an ice cover of 80% or greater for 74 days.  No other winter had more than 52 such days.

According to this NOAA file in the winter of 2013-14 Lake Superior has an ice cover of 80% or greater on 68 days.  

Assel also studied Lake Erie, and finds that 1911-12 had 72 days with 80% or greater ice cover.  This compares to 70 such days in 2013-14.

Based on the above, it is very likely that 1911-12 was the most recent winter with similar April ice cover to 2013-14.