The flow of the river is greatest in the spring, when heavy rainfall and melting snow on the tributaries (especially the Missouri and the Ohio) cause the main stream to rise and frequently overflow its banks and levees, inundating vast areas of the plain. Since the disastrous flood of 1927 the U.S. Congress has authorized the construction of dams on the upper Mississippi and its tributaries to regulate the flow; the building of c.1,600 mi (2,580 km) of levees below Cape Girardeau to contain the swollen river; and the establishment of floodways to divert water at critical points, such as the Cairo–New Madrid, Atchafalaya, and Morganza floodways and the Bonnet Carre Spillway at New Orleans, which diverts water into Lake Pontchartrain. Cutoffs have eliminated the dangerous winding channels, and an improved main channel has increased the river's flood-carrying capacity. A 220-acre (89-hectare) model of the Mississippi River basin is located at Clinton, Miss., which has been used by the U.S. Corps of Engineers to simulate various conditions in the basin.
Nonetheless, serious, record-breaking floods again occurred in the rainy spring of 1973, when the river crested at St. Louis at 43.3 ft (13.2 m), and again in the summer of 1993, when the river crested at St. Louis at 49.6 ft (15.1 m), killing 50 people, displacing 50,000, and causing $12 billion in agricultural and property damage. In the spring of 2011, heavy rains in April in the S central Mississippi river basin led to near-record high water and flooding from parts of Missouri and Illinois south. The narrow river channel that has been created by building levees has worsened flooding in some instances.
In 1988 a severe drought brought water levels down to their lowest point in recorded history and halted most river traffic. Severe drought again threatened to halt traffic in the middle Mississippi in 2012–13, but dredging and other channel deepening measures kept the river open.