The Mississippi River is called the Lower Mississippi River

The Mississippi River is called the Lower Mississippi River from its confluence with the Ohio River to its mouth at the Gulf of Mexico. Sony VAIO PCG-51112M Battery

Measured by water volume, the Lower Mississippi's primary branch is the Ohio River. At the confluence of the Ohio and the Middle Mississippi, the Ohio is the bigger river, with its long-term mean discharge at Cairo, Illinois being 281,500 cu ft/s (7,970 m³/s),while the long-term mean discharge of the Mississippi at Thebes, Sony VAIO PCG-51211M Battery

Illinois (just upriver from Cairo) is 208,200 cu ft/s (5,900 m³/s).^[14] Thus, by volume, the main branch of the Mississippi River system at Cairo can be considered to be the Ohio River (and the Allegheny River further upstream), rather than the Middle Mississippi. Sony VAIO PCG-51212M Battery

In addition to the Ohio River, the major tributaries of the Lower Mississippi River are the White River, flowing in at theWhite River National Wildlife Refuge in east central Arkansas; the Arkansas River, joining the Mississippi at Arkansas Post; the Big Black River in Mississippi; Sony VAIO PCG-41111M Battery

the Yazoo River, meeting the Mississippi at Vicksburg, Mississippi; and the Red River in Louisiana. The widest point of the Mississippi River is in the Lower Mississippi portion where it exceeds 1 mile (1.6 km) in width in several places.

Deliberate water diversion at the Old River Control Structure in Louisiana allows Sony VAIO PCG-41111V Battery

the Atchafalaya River in Louisiana to be a major distributary of the Mississippi River, with 30% of the Mississippi flowing to the Gulf of Mexico by this route, rather than continuing down the Mississippi's current channel pastBaton Rouge and New Orleans on a longer route to the Gulf. Sony VAIO PCG-61412V Battery

The Mississippi River has the world's fourth largest drainage basin ("watershed" or "catchment"). The basin covers more than 1,245,000 sq mi (3,220,000 km²), including all or parts of 32 U.S. states and two Canadian provinces. The drainage basin empties into the Gulf of Mexico, part of the Atlantic Ocean. Sony VAIO PCG-71112M Battery

The total catchment of the Mississippi River covers nearly 40% of the landmass of the continental United States.In the United States, the Mississippi River drains the majority of the area between crest of the Rocky Mountains and the crest of the Appalachian Mountains, except for various regions drained to Hudson Bay by the Red River of the North; Sony VAIO PCG-81111V Battery

to the Atlantic Ocean by the Great Lakes and the Saint Lawrence River; and to the Gulf of Mexico by the Rio Grande, the Alabama andTombigbee rivers, the Chattahoochee and Appalachicola rivers, and various smaller coastal waterways along the Gulf. Sony VAIO PCG-81111V Battery

The Mississippi River empties into the Gulf of Mexico about 100 miles (160 km) downstream from New Orleans. Measurements of the length of the Mississippi from Lake Itasca to the Gulf of Mexico vary somewhat, but the United States Geological Survey's number is 2,340 miles (3,770 km). Sony VAIO PCG-81212M Battery

The retention time from Lake Itasca to the Gulf is typically about 90 days.^[18]

The Mississippi River discharges at an annual average rate of between 200 and 700 thousand cubic feet per second (7,000–20,000 m³/s).^[19] Although it is the 5th largest river in the world by volume, Sony VAIO PCG-81212V Battery

this flow is a mere fraction of the output ofthe Amazon, which moves nearly 7 million cubic feet per second (200,000 m³/s) during wet seasons. On average, the Mississippi has only 8% the flow of the Amazon River.^[20]

Fresh river water flowing from the Mississippi into the Gulf of Mexico does not mix into the salt water immediately. Sony VAIO PCG-51111M Battery

The images from NASA's MODIS (to the right) show a large plume of fresh water, which appears as a dark ribbon against the lighter-blue surrounding waters. These images demonstrate that the plume did not mix with the surrounding sea water immediately. Sony VAIO VPCS13X9E/B battery

Instead, it stayed intact as it flowed through the Gulf of Mexico, into the Straits of Florida, and entered theGulf Stream. The Mississippi River water rounded the tip of Florida and traveled up the southeast coast to the latitude ofGeorgia before finally mixing in so thoroughly with the ocean that it could no longer be detected by MODIS. Sony VAIO VPCS12V9E/B battery

Prior to 1900, the Mississippi River transported an estimated 400 million metric tons of sediment per year from the interior of the United States to coastal Louisiana and the Gulf of Mexico. During the last two decades, this number was only 145 million metric tons per year. Sony VAIO VPCS12V9E/B battery

The reduction in sediment transported down the Mississippi River is the result of engineering modification of the Mississippi, Missouri, and Ohio rivers and their tributaries by dams, meander cutoffs, river-training structures, and bank revetments and soil erosion control programs in the areas drained by them.^[21] Sony VAIO VPCF13M8E/B battery

Over geologic time, the Mississippi River has experienced numerous large and small changes to its main course, as well as additions, deletions, and other changes among its numerous tributaries, and the lower Mississippi River has used different pathways as its main channel to the Gulf of Mexico across the delta region. Sony VAIO VPCF13Z0E/B battery

Through a natural process known as avulsion or delta switching, the lower Mississippi River has shifted its final course to the mouth of the Gulf of Mexico every thousand years or so. This occurs because the deposits of silt and sediment begin to clog its channel, raising the river's level and causing it to eventually find a steeper, more direct route to the Gulf of Mexico. Sony VAIO VPCM13M1E/L battery

The abandoned distributaries diminish in volume and form what are known as bayous. This process has, over the past 5,000 years, caused the coastline of south Louisiana to advance toward the Gulf from 15 to 50 miles (25–80 km). The currently active delta lobe is called the Birdfoot Delta, after its shape, or the Balize Delta, after La Balize, Louisiana, the first French settlement at the mouth of the Mississippi. Sony VAIO VPCM13M1E/W battery

The current form of the Mississippi River basin was largely shaped by the Laurentide Ice Sheet of the most recent Ice Age. The southernmost extent of this enormous glaciation extended well into the present-day United States and Mississippi basin. Sony VAIO VPCF22M1E battery

When the ice sheet began to recede, hundreds of feet of rich sediment were deposited, creating the flat and fertile landscape of the Mississippi Valley. During the melt, giant glacial rivers found drainage paths into the Mississippi watershed, creating such features as the Minnesota River, James River, and Milk River valleys. Sony VAIO VPCF11M1E/H battery

When the ice sheet completely retreated, many of these "temporary" rivers found paths to Hudson Bay or the Arctic Ocean, leaving the Mississippi Basin with many features "oversized" for the existing rivers to have carved in the same time period.

Ice sheets during the Illinoian Stage about 300,000 to 132,000 years before present, Sony VAIO VPCF13M0E/B battery

blocked the Mississippi near Rock Island, Illinois, diverting it to its present channel farther to the west, the current western border of Illinois. The Hennepin Canal roughly follows the ancient channel of the Mississippi downstream from Rock Island to Hennepin, Illinois. Sony VAIO VPCYB2M1E battery

South of Hennepin, to Alton, Illinois, the current Illinois River follows the ancient channel used by the Mississippi River before the Illinoian Stage.

In March 1876, the Mississippi suddenly changed course near the settlement of Reverie, Tennessee, leaving a small part ofTipton County, Sony VAIO VPCYB3V1E battery

Tennessee, attached to Arkansas and separated from the rest of Tennessee by the new river channel. Since this event was an avulsion, rather than the effect of incremental erosion and deposition, the state line remains located in the old channel.^[22]

Geologists consider the next major change in the course of the Lower Mississippi now overdue. Sony VAIO VPCY11M1E battery

Either of two new routes – through the Atchafalaya Basin or through Lake Pontchartrain — might become the Mississippi's main channel if flood-control structures are overtopped or heavily damaged during a severe flood. However, the Mississippi returned to its present course following the great floods in 2011. Sony VAIO VPCS12L9E/B battery

Failure of the Old River Control Structure, the Morganza Spillway, or nearby levees would likely re-route the main channel of the Mississippi through Louisiana's Atchafalaya Basin and down the Atchafalaya River to reach the Gulf of Mexico south of Morgan City in southern Louisiana. Sony VAIO VPCF11S1E/B battery

This route provides a more direct path to the Gulf of Mexico than the present Mississippi River channel through Baton Rouge and New Orleans.^[25] While the risk of such a diversion is present during any major flood event (such as those of 1973 or 2011), such a change has so far been prevented by active human intervention involving the construction, Sony VAIO VPCYB3V1E/R Battery

maintenance, and operation of various levees, spillways, and other control structures by the U.S. Army Corps of Engineers.The Old River Control Structure, between the present Mississippi River channel and the Atchafalaya Basin, sits at the normal water elevation and is ordinarily used to divert 30% of the Mississippi's flow to the Atchafalaya River. Sony VAIO VPCF23P1E Battery

There is a steep drop here away from the Mississippi's main channel into the Atchafalaya Basin. If this facility were to fail during a major flood, there is a strong concern the water would scour and erode the river bottom enough to capture the Mississippi's main channel. Sony VAIO VPCF23N1E Battery

The structure was nearly lost during the 1973 flood, but repairs and improvements were made after engineers studied the forces at play. In particular, the Corps of Engineers made many improvements and constructed additional facilities for routing water through the vicinity. Sony VAIO VPCY21S1E/L Battery

These additional facilities give the Corps much more flexibility and potential flow capacity than they had in 1973, which further reduces the risk of a catastrophic failure in this area during other major floods, such as those of 2011.

Because the Morganza Spillway is located at slightly higher elevation well back from the river, Sony VAIO VPCY21S1E/G battery

it is normally dry on both sides.^[28] Even if this structure were to fail at the crest during a severe flood, the flood waters would have to cause a significant amount of erosion, down to normal water levels, before the Mississippi could permanently jump channel at this location. Sony VAIO VPCF24M1E battery

During the 2011 floods, the Corps of Engineers decided to open the Morganza Spillway to 1/4 of its capacity to allow 150,000 ft³/sec of water to flood the Morganza and Atchafalaya floodways and continue directly to the Gulf of Mexico, bypassing Baton Rouge and New Orleans. Sony VAIO VGN-AW110J/H Battery