Ice and snow help slow evaporation of water on Great Lakes. Ice slows commercial traffic but it also means waters levels will be up.

“A brutally cold winter has covered the Great Lakes with more ice than they have seen since 1979. Special correspondent Elizabeth Bracket of WTTW reports on the struggle to keep shipping lanes open to Chicago’s ice-clogged harbor to Lake Michigan.”

http://www.pbs.org/newshour/bb/coping-with-exceptional-ice-great-lakes/