During the retreat of the last ice age isostatic rebound occurred propagating like a damped wave parallel to the ice front. Beaches of large glacial lakes show this clearly. In areas where such lakes were too short lived to develop well defined beaches evidence of rebound must be sought elsewhere. In regions where the general slope of the land is towards the retreating ice front, rebound will trigger incision of the down valley section. If a bed is uplifted which is more resistant than the rate of incision, aggradation will occur in an upstream section of the river. This mechanism seems to best explain the fill in the South Saskatchewan River Valley at Elbow. In this case a bouldery till near Saskatoon appears to behave as a resistant layer. Minimum deformation derived from the evidence available is comparable to that measured on Lake Agassiz shorelines.