Sinking land, called subsidence, is caused by human activities such as mining and extraction of underground fluids, like petroleum, natural gas, or groundwater. Under normal conditions, deltas form where large rivers enter the ocean. The rivers carry silt, gravel and clay downstream. When the river reaches the shallow coast, the water moves slower, which allows these sediments to accumulate.
The deposits build up land, creating a river delta. Deltas are important marine environments and diverse ecosystems. Plants thrive in their rich soil.
The habitat provides food and a nursery for fish, crustaceans, and mammals. 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, 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, 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, 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 , acres of land and shallow estuarine water area in the active Mississippi River delta.
Approximately 83 percent of this area, or , acres, is open water. The , 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, 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 , cubic feet per second cfs. A peak discharge of approximately 1,, cfs occurs on the average of once every 16 years downstream of New Orleans. Suspended sediment concentrations in the river decreased markedly between and Since that time the observed decrease in the suspended sediment load has been minimal.
Long-term suspended sediment loads in the river average , tons per day; they have ranged from an average of 1,, tons per day in to a still considerable average of , tons per day in Between and the land loss rate in the Mississippi River Delta Basin averaged 1, acres per year, or 1. Between the mid's and , the estimated land loss rate for the basin was 2, 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 , 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. The Rights Holder for media is the person or group credited. For information on user permissions, please read our Terms of Service. If you have questions about how to cite anything on our website in your project or classroom presentation, please contact your teacher.
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June 10, TIFF. October 15, JPEG. October 15, TIFF. On June 10, , Landsat 1 acquired the top image. On October 10, , Landsat 7 acquired the bottom image.
In the years that separated these acquisitions, the river delta saw retreating shorelines, inundations, and some additional new land. In the image, new land appears most conspicuously along the southwestern tip of the delta, on the southeast side of the waterway.
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