Mississippi River Delta Basin
The Mississippi River has had a profound
effect on the landforms of coastal Louisiana. The entire area is the product of
sediment deposition following the latest rise in sea level about 5,000 years
ago. Each Mississippi River deltaic cycle was initiated by a gradual capture of
the Mississippi River by a distributary which offered a shorter route to the
Gulf of Mexico. After abandonment of an older delta lobe, which would cut off
the primary supply of fresh water and sediment, an area would undergo
compaction, subsidence, and erosion. The old delta lobe would begin to retreat
as the gulf advanced, forming lakes, bays, and sounds. Concurrently, a new delta
lobe would begin its advance gulfward. This deltaic process has, over the past
5,000 years, caused the coastline of south Louisiana to advance gulfward from 15
to 50 miles, forming the present-day coastal plain.
For the last 1,200 years, sediment deposition has occurred primarily at the mouth of the Mississippi River's Plaquemines-Balize delta, in the area defined as the Mississippi River Delta Basin. This delta is located on the edge of the continental shelf of the Gulf of Mexico. Its bird's foot configuration is characteristic of alluvial deposition in deep water. In this configuration large volumes of sediment are required to create land area; consequently, land is being lost in this delta more rapidly than it is being created.
The Mississippi River Delta Basin comprises approximately 521,000 acres of land and shallow estuarine water area in the active Mississippi River delta. Approximately 83 percent of this area, or 420,000 acres, is open water. The 101,100 acres of land in the basin are characterized by low relief, with the most prominent features being natural channel banks and dredged material disposal areas along the Mississippi River, its passes, and man-made channels. Coastal marshes make up approximately 61,650 acres or about 61 percent of the total land area in the Mississippi River Delta Basin. Eighty-one percent of this marsh is fresh, 17 percent is intermediate, and 2 percent is brackish-saline.
The Mississippi River discharges the headwater flows from about 41 percent of the contiguous 48 states. On a long-term daily basis, discharges in the Mississippi River average 470,000 cubic feet per second (cfs). A peak discharge of approximately 1,250,000 cfs occurs on the average of once every 16 years downstream of New Orleans.
Suspended sediment concentrations in the river decreased markedly between 1950 and 1966. Since that time the observed decrease in the suspended sediment load has been minimal. Long-term suspended sediment loads in the river average 436,000 tons per day; they have ranged from an average of 1,576,000 tons per day in 1951 to a still considerable average of 219,000 tons per day in 1988.
Between 1974 and 1990 the land loss rate in the Mississippi River Delta Basin averaged 1,072 acres per year, or 1.69 percent of existing land area (Dunbar, Britsch, and Kemp 1992). Between the mid-1950's and 1974, the estimated land loss rate for the basin was 2,890 acres per year. This loss is the result of compaction, subsidence, hurricanes, tidal erosion, sea level rise, and human activities. The loss has been aggravated by maintenance of navigation channels and construction of canals for mineral exploration. The total land area lost in this basin over the last 60 years has been approximately 113,300 acres.
The primary wetlands loss problem facing the Mississippi River Delta Basin is that of subsidence and compaction. Unlike other areas of coastal Louisiana, the Mississippi River delta is blessed with a relative abundance of inflowing fresh water and sediments. Despite the availability of these resources, the overall growth of emergent delta has been truncated in recent history. In its present position the Mississippi River deposits sediments into much deeper water than has been the case historically. This is evidenced by the thick stratum of Holocene deltaic sediments found in the active river delta. These unconsolidated sediments are highly susceptible to compaction, reducing the life span of emergent wetlands. While the rapid emergence of wetlands can occur over large areas in the delta, these areas deteriorate in an equally rapid manner.
Human activities have aggravated land loss rates in the Plaquemines-Balize delta. The stabilization of the Mississippi River's channel has cut off seasonal sediment-laden overbank flow that once nourished adjacent wetland areas. The Mississippi River levees to the north, and associated erosion control and channel stabilization measures extending to its mouth, also preclude the possibility of a naturally occurring crevasse or change in the river's course.
Many areas of the Louisiana coast suffer from a lack of the abundant fresh water and sediment found in the Mississippi River. Since the river is no longer free to alter its course and leave its banks to inundate vast coastal areas, the effects of human and natural forces which promote wetland deterioration are compounded. In this respect the relationship between the Mississippi River and the problems facing coastal wetlands is not limited to the river's delta, but extends across the entire Louisiana coast. The lack of growth in the Mississippi River delta, on a large scale, is as much a coast-wide problem as a basin problem. This source of ample fresh water and sediment, which shaped the Louisiana coast as we know it, is no longer producing a net gain in coastal wetlands, placing the entire Louisiana coast at risk.
Mississippi River Delta Basin Summary
Mississippi River Delta Basin Discussion
Mississippi River Basin Dynamics
Basin-Wide Land Loss Map for
Mississippi River Delta Basin. |
Basin-Wide Habitats Maps for
Mississippi River Delta Basin. |
(Time lapsed animation of basin from 1956 to 1993.)
CWPPRA Restoration Sites for the Mississippi River Delta Basin
| PPL | Number | Agency | Project Name |
| 4 | MR-08 | COE | Beneficial Use of Hopper Dredged Material Demonstration (Deauthorized) |
| 10 | MR-13 | COE | Benneys Bay Diversion |
| 3 | MR-06 | COE | Channel Armor Gap Crevasse |
| 6 | MR-09 | NMFS | Delta Wide Crevasses |
| 6 | MR-10 | COE | Dustpan Maintenance Dredging Operations for Marsh Creation in the Mississippi River Delta Demonstration |
| 12 | MR-12 | COE | Mississippi River Sediment Trap |
| 3 | MR-07 | COE | Pass-a-Loutre Crevasse (Deauthorized) |
| 9 | MR-11 | COE | Periodic Introduction of Sediment and Nutrients at Selected Diversion Sites Demonstration |
| 13 | MR-14 | COE | Spanish Pass Diversion |
| 15 | MR-15 | COE | Venice Ponds Marsh Creation and Crevasses |
| 1 | MR-03 | COE | West Bay Sediment Diversion |
11 projects listed
PPL stands for "Priority Project List"