There are many reasons for Great Lakes water levels to rise.  In Lake Michigan, a primary reason is the balance between evaporation and precipitation.  Back-to-back cold winters in 2013 and 2014 contributed to the sudden increase in 2014.  Ice cover blocked typical patterns of winter evaporation and then contributed to subsequent runoff from melting of snow and ice. Stronger storms and unusually wet years since then are pushing Great Lakes water levels up.