The Red River is part of the system of waterways created when the glacial lake called Lake Agassiz retreated. The shallow valley and the vast expanse of very flat terrain beyond the river basin cannot hold the Red River's volume at times of very high water levels. When the river overflows its banks, many square kilometres of farmland are at risk of flooding. The greatest flood ever recorded in the area occurred in 1826. The entire population of the Red River Settlement, then some 400 souls, had to be evacuated. It was only the first of several floods to have a major impact on the region.