A freighter trapped in ice is shown in this aerial photo near Whitefish Bay on Lake Superior northwest of Sault Ste. Marie, Ontario April 7, 2015. The biggest ice cover on the Great Lakes in decades is backing up shipments of everything from Canadian grain to U.S. iron and steel in one of North America's most important economic regions. REUTERS/Kenneth Armstrong 

The Canadian Coast Guard Ship (CCGS) Samuel Risley is shown in this aerial photo near Whitefish Bay on Lake Superior northwest of Sault Ste. Marie, Ontario April 7, 2015. The icebreaker was participating in an effort to free eight freighters from heavy ice in eastern Lake Superior as the biggest ice cover on the Great Lakes in decades is backing up shipments of everything from Canadian grain to U.S. iron and steel in one of North America's most important economic regions. REUTERS/Kenneth Armstrong 

The Canadian Coast Guard Ship (CCGS) Samuel Risley is shown in this aerial photo near Whitefish Bay on Lake Superior northwest of Sault Ste. Marie, Ontario April 7, 2015. The icebreaker was participating in an effort to free eight freighters from heavy ice in eastern Lake Superior as the biggest ice cover on the Great Lakes in decades is backing up shipments of everything from Canadian grain to U.S. iron and steel in one of North America's most important economic regions. REUTERS/Kenneth Armstrong 

Freighters trapped in ice are shown in this aerial photo near Whitefish Bay on Lake Superior northwest of Sault Ste. Marie, Ontario April 7, 2015. The biggest ice cover on the Great Lakes in decades is backing up shipments of everything from Canadian grain to U.S. iron and steel in one of North America's most important economic regions. REUTERS/Kenneth Armstrong 

The James R Barker Lake Freighter is shown trapped in ice in this aerial photo near Whitefish Bay on Lake Superior northwest of Sault Ste. Marie, Ontario April 7, 2015. The biggest ice cover on the Great Lakes in decades is backing up shipments of everything from Canadian grain to U.S. iron and steel in one of North America's most important economic regions. REUTERS/Kenneth Armstrong 

Freighters trapped in ice are shown in this aerial photo near Whitefish Bay on Lake Superior northwest of Sault Ste. Marie, Ontario April 7, 2015. The biggest ice cover on the Great Lakes in decades is backing up shipments of everything from Canadian grain to U.S. iron and steel in one of North America's most important economic regions. REUTERS/Kenneth Armstrong 

United States Coast Guard ships are shown in this aerial photo near Whitefish Bay on Lake Superior northwest of Sault Ste. Marie, Ontario April 7, 2015. The icebreakers are participating in an effort to free eight freighters from heavy ice in eastern Lake Superior as the biggest ice cover on the Great Lakes in decades is backing up shipments of everything from Canadian grain to U.S. iron and steel in one of North America's most important economic regions. REUTERS/Kenneth Armstrong 

The John G. Munson Lake Freighter is shown trapped in ice in this aerial photo near Whitefish Bay on Lake Superior northwest of Sault Ste. Marie, Ontario April 7, 2015. The biggest ice cover on the Great Lakes in decades is backing up shipments of everything from Canadian grain to U.S. iron and steel in one of North America's most important economic regions. REUTERS/Kenneth Armstrong 

The Canadian Coast Guard Ship (CCGS) Samuel Risley (L) is shown in this aerial photo near Whitefish Bay on Lake Superior northwest of Sault Ste. Marie, Ontario April 7, 2015. The icebreaker was participating in an effort to free eight freighters from heavy ice in eastern Lake Superior as the biggest ice cover on the Great Lakes in decades is backing up shipments of everything from Canadian grain to U.S. iron and steel in one of North America's most important economic regions. REUTERS/Kenneth Armstrong