sentences:  The resulting enormous outflow of the lake carved a natural spillway through the moraine, through which cascaded Glacial River Warren.This great river not only created the gap, it also cut the valleys of the present-day Minnesota and Upper Mississippi Rivers.River Warren drained Agassiz in twice more over the next 2300 years, separated by intervals when the ice sheet receded sufficiently to uncover other outlets for Lake Agassiz.