A theory postulates that a huge ice lake named “Lake Agassiz” existed near the border between Canada and the United States during the last glacial age. It was thought that this lake collapsed, sometime between 13,000 to 8,200 years ago, causing global environmental changes such as cooling in the Younger Dryas period and the sea level rise. However, the truth about Lake Agassiz and its collapse remained unclear. To verify the actuality of the collapse, a simulation software was created for the geomorphological analysis and water volume calculation. The result of the present analysis indicated that the amount of water in Lake Agassiz was much smaller than presumed in the previous theory. Considering the surrounding topography, we deduced that Lake Agassiz was not the type of lake whose collapse would have caused a large-scale flood. Additionally, from a slope map created for the North American continent, we discovered a topography that appears to be a trace of erosion caused by a large-scale flood near Lake Agassiz. These findings reveal that the flooding of Lake Agassiz was likely caused by the collapse of an even larger ice-sheet lake. This study considers the scale and mechanism of the floods from a giant ice-sheet lake that existed in the Laurentide Ice Sheet.