The Minneapolis Star-Tribune explains the geology of the Flood of 2009.
The Red River of the North flows along the bed of an ancient glacial lake, Lake Agassiz that covered northern Minnesota and stretched up into Manitoba. The bed is flat and the river meanders across it in many tight loops.
As the Wisconsinan ice sheet withdrew from northern Minnesota Glacial Lake Agassiz formed behind a moraine that separates drainage to the Arctic and drainage to the Gulf of Mexico along the Minnesota River and Mississippi. Glacial ice prevented the lake from draining to the north and to the east through the Great Lakes. The lake filled to overflowing and spilled over the moraine into the Minnesota River and then the Mississippi. We call the glacial flood River Warren and it set the course of the modern Mississippi.
When the ice sheet retreated some more and opened up drainage to the east, River Warren expired and Glacial Lake Agassiz drained to the great lakes. Eventually, the ice retreated enough so that Agassiz drained to the Arctic, leaving behind its very flat lake bed, across which the Red River flows in very tight meanders. So do the Buffalo, the Goose, the Wild Rice, the Sheyenne, the Maple. They are all flow across the ancient lake bed and they are all in flood.
Snow melt and flooding at the southern end of the Red River drainage causes problems for the cities along the river, because the river is still frozen in its northern reaches. The water has no place to go until the northern ice melts.