Ice covers on rivers are stressed in the downstream direction mainly by the friction of the under flow and the effect of the slope of the water line. Analysis show that in wide rivers, these stresses are the main factor governing the equilibrium thickness of the cover. This analysis allows forecasting of ice covers evolution, including the formation and size of ice jams, losses of head and ice thrust on retaining booms. Laws of similitude derived from the basic equations allow the operation of scale models when the complexity of the topography renders numerical calculation Impossible. Description of several practical applications shows good agreement between model or theoretical prediction and prototype measurements.