Mississippi drainage basin Geography of the Interior United States

the missouri river (center) joins upper mississippi river (right), more doubling flow south (left). wood river, illinois in foreground.

the small proportion of total water volume supplied great missouri river basin due light precipitation in region. lower mississippi has no large tributaries lower east, 2 important ones come west. mississippi arkansas drainage area being little less ohio river , basin of red river of louisiana being half large. mississippi river drains area of one-third of united states. head of coastal plain embayment near junction of ohio , mississippi. flows southward 560 miles (900 km) through semi-consolidated strata of plain. river has eroded valley 40 50 miles (64 80 km) wide enclosed bluffs 1 or 2 hundred feet high in northern part. these bluffs decrease towards south, local increase of height associated decrease in flood plain breadth on eastern side grand gulf escarpment traversed.

this valley in coastal plain, narrower rock-walled valley of upper river in prairie states, true valley of mississippi river. however, in popular usage, mississippi valley taken include large central part of mississippi drainage basin.

the valley floor covered floodplain of fine silt, having southward slope of half foot mile (100 mm/km). length of river itself, ohio mouth gulf 1,060 miles (1,710 km) due windings. mean fall 3 inches per mile (50 mm/km). on account of rapid deposition of sediment near main channel @ times of overflow, flood plain, case on mature valley floors, has lateral slope of as 5, 10, or 12 ft (3.7 m) in first mile river, decreases less amount. short distance river, flood plain swampy, unless surface there aggraded tributary streams. reason louisiana, arkansas , mississippi rank after florida in swamp area.

the great river receives abundant load of silt tributaries, , takes , lays down silt own bed , banks every change of velocity. swiftest current follows outer side of every significant curve in channel. concave bank on side of fastest part of river worn away. chance irregularity exaggerated, , in time series of large serpentines or meanders developed, most-symmetrical examples @ present being near greenville, mississippi. growth of meanders tends give river continually increasing length. tendency counteracted sudden occurrence of cut-offs time time, constant length maintained.

the floods of mississippi occur in spring or summer. owing great size of drainage basin, seldom happens 3 upper tributaries simultaneously flooded. serious problem lower river if 2 of large tributaries flood @ same time. in case, lower river rise 30, 40 or 50 feet (15 m). fall of river steepened , velocity accelerated down stream point of highest rise. conversely, fall , velocity both diminished stream same point.

the load of silt carried down stream river finally, after many halts on way, reaches waters of gulf. there, decrease of velocity aided salinity of sea water, causes formation of remarkable delta, leaving less aggraded areas shallow lakes (lake pontchartrain on east, , grand lake on west of river). ordinary triangular form of deltas, due smoothing of delta front sea action, here wanting, because of weakness of sea action in comparison strength of current in each of 4 distributaries or passes river divides near mouth.