The Mississippi Delta Region

Perrin H. Grissom.

The Mississippi Delta region is the alluvial plain of the Mississippi Valley. It stretches from Cape Girardeau, Mo., to the gulf coast, and it includes the flood plain, the deltaic plain, and the loessial terraces.

The elevation increases northward from sea level at the Gulf of Mexico to about 320 feet above sea level in Missouri. The loessial terraces rise 20 to 50 feet higher than the surrounding flood plain. The flood plain consists of low meander belts of ridges and intervening irregular flood basins.

Most of the surface features reflect the depositional activities of running water. The ridges and basins of the upper part of the deltaic plain are somewhat more uniform than the flood plain. The ridges stand higher above the general surface. The lowlands often are covered with water. The lowlands between the ridges give way to coastal marshes and bays, and the marsh areas are characterized by wave action. The loessial terraces are the portions of the alluvial fans of the tributary streams that stand above the general level of the flood plain and are blanketed with loesslike deposits.

The entire alluvial plain is slightly more than 650 miles long. It is 25 miles wide between Natchez, Miss., and Sicily Island, La., and 125 miles wide in the latitude of Helena, Ark. The average width is about 70 miles.

The land area in the Mississippi Valley comprises approximately 27 million acres. About 11.5 million acres of the total remain in forest.

The soil types reflect the action of floodwater. The soils vary from clay to sand and have poor to excessive internal drainage.

At least 30 States have contributed, through erosion, to the soils of the Mississippi Valley. The constant migration and shifting of the channels of the Mississippi and Ohio Rivers in earlier times and the overflows from many tributaries have largely determined the pattern of the surface soils. Alternation of silts, sands, and clays is characteristic of an alluvial profile.

The soil types along the old stream channels generally are fine sandy loams or loams. The broad, flat basins are mostly clay soils, and the intermediate areas, between the basins and the ridges, are silt loams and silty clay soils. As streams overflowed, the sand particles settled out along the stream. The silt and clay particles were held in suspension much longer and finally were deposited by relatively still water in broad, flat areas. The areas along the old stream channels occupied by the coarser textured soils generally rise 5 to 15 feet above the adjacent basins.

Just as water has been the chief factor in the formation of soils in the Mississippi alluvial plain, its management is a major factor in the management of soils.

The stream channels until recent years were inadequate to carry the waterload, and large areas were flooded frequently. Damage to crops and property was tremendous, but the alluvial deposits maintained and replenished an already fertile soil. The deposition usually was beneficial to the soils, but the floods and threat of floods made agriculture hazardous.

The construction of an elaborate levee system and the straightening of stream channels have reduced the possibilities of floods. It has become necessary to base the soil-management program on the soils on hand; further alteration by floodwaters should no longer be anticipated.

Probably in no area of the United States is the conflict between too much water and too little water as pronounced as in the Mississippi Delta region. Excess surface water seriously handicaps winter-growing crops in winter and spring, interferes with land preparation for spring-planted row crops, and often retards the growth of the young plants.

The total annual rainfall in the region is more than the amount required for optimum plant growth, but the distribution is such that water often is scarce in summer. Therefore drainage must be provided, and consideration must be given at the same time to the conservation of water.

Surface drainage, as such, is not necessarily a function of soil management, but the lack of it or the manner of achieving it may vitally influence soil management.

The need for drainage has long been recognized in this region. Lack of drainage has posed a constant threat to stability of yields and has been the chief cause of crop failure.

We can consider drainage as primary and secondary, or farm, drainage.

Primary drainage has reference to drainage in districts or local watersheds. It has been attempted generally through the organized efforts of landowners in a community or district. Such efforts have improved drainage immeasurably in some districts. A hodgepodge of districts has been organized, however, with little or no coordination among them. They have solved local problems, but new problems frequently have been created for other localities downstream.

Primary drainage must be achieved on a broad basis with more comprehensive planning. A successful primary drainage program will have its greatest impact on lands already in cultivation by removing excess water from them. It is doubtful whether the elimination of excess water will benefit land that is not already planted to cultivated crops.

Most of the soils remaining in forest were formed from slack-water sediments and have poor internal drainage. The removal of the excess surface water cannot be expected to affect greatly the natural soil characteristics, and these soils cannot be expected to become highly productive if farmers grow crops that require good aeration and good internal drainage.

Primary drainage is a prerequisite to field drainage, but field drainage is associated more closely with soil management.

Field drainage is necessary, yet the farm effort to provide field drainage in the Mississippi alluvial plain, especially in the loessial terrace areas, has been one major factor contributing to erosion, a poor physical condition of the soil, and a water shortage during periods of greatest need.

Surface drainage of fields may be achieved in two ways. One way is to provide enough structures to remove the water from each low place. The other is to arrange the rows to fit the land so that excess water may be removed but slowly enough to minimize erosion. Grading and smoothing is necessary to provide the base for proper row alinement on much land.

Drainage by elevation also has been tried. Ridges and low places have been created artificially through the tillage methods. The result usually has been overdrainage of the high places and an extension of the poorly drained low places.