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Surface- and Groundwater Hydrology of the Acadian-Pontchartrain NAWQA
A map of streams and waterbodies with elevation.
Click on the image for a much larger version in GIF format.
You may also view a large PNG version of the above graphic.
Lower Mississippi Delta and Wet Prairie Streams
Lower Mississippi Delta and Wet Prairie streams form a complex network across southern Louisiana. These streams are naturally low-gradient, muddy and highly turbid, with moderate alkalinity and ionic strength (dissolved salts). Many streams have natural connections between them such as distributaries and abandoned meanders, or manmade canals, creating complex drainage patterns. Small to medium-sized streams tend to have naturally deep, U-shaped beds with sticky clay substrates. Larger rivers may develop layers of water similar to conditions found in lakes.
Streams here tend to have very slow flow, but water can also be pushed upstream by tides or wind. These generally stagnant, backwater conditions allow the dense growth of aquatic plants, forming a large area for animals to hide in. Woody debris also tends to be abundant in these streams, providing a hard surface to cling to. Because of the very slow water, habitat conditions often resemble those of shallow lakes and ponds. Swamps and marshes develop naturally in poorly drained areas, such as in between bayous where their levees form a "bowl" (floodplain backwaters), or seasonally inundated floodplains.
Streams in the mixed pine-forest uplands have a moderate flow gradient and sandy, shifting beds that are reshaped quickly in the fast water that is usual for flood conditions. Habitats for algae and invertebrates exist where the water is slowed down or on hard surfaces where they can cling. These include deep pools, undercut banks, fallen trees and woody debris. Fish also find shelter in these areas.
Water in these streams has low alkalinity and ionic strength (fewer dissolved salts and other ions), but tends to have much higher dissolved oxygen. Because the bed sediment is largely sand, the water is fairly clear, except where there are nearby areas of soil erosion.
Water Resource Uses
In the southernmost parts of Louisiana, drinking water is supplied from surface, rather than ground-water. Along the coast, surface water is the only source for fresh water. Large amounts of agrichemicals and volatile organic chemicals can pose problems for the drinking supply here.
Agriculture and Industry
Surface water is pumped to irrigate fields (or flood them for rice). In the Western Prairies, the amount of pumping can be sufficient to cause flow reversals in some smaller streams. In coastal areas, industries rely on surface water rather than ground water. Many industries locate along the Mississippi River for the large amounts of fresh water that are readily available.
Shipping and Transportation
Industrial activities such as shipbuilding, shipping, fishing, petrochemistry, and oil rigs occur along many bayous and rivers in or near urban centers, or near the coast for easy access to open water. Natural waterbodies and man-made canals are often dredged to maintain enough depth for the movement of large ships.
Modifications to Flow:
Levees are naturally created in the process of floods, where sediment will drop out of floodwater immediately next to the waterbody. Levees along many bayous and rivers have been built up higher by people to reduce floods and to maintain a deeper channel for shipping. Levees are also constructed for coastal communities to protect against hurricane storm surge.
Canals and Drainage
Canals and ditches are constructed to increase drainage for agriculture and urban development, or to redirect potential floodwaters away from developed areas. Other canals are constructed to reduce distances for shipping (which must otherwise travel miles of river meanders), and are easier for barges and other large ships to navigate.
Other Controls on Flow
The Old River Control Structure sits at the confluence of the Mississippi, Red, and Atchafalaya Rivers to cirect the majority of flow through the Mississippi. In uncontrolled conditions, the Atchafalaya would "capture" the majority of Mississippi and Red River flows because of its slightly steeper gradient and more direct route to the ocean. This would greatly reduce the amount of water travelling down the Mississippi, which would constrain drinking water sources and shipping.
Emergency Mississippi flood outlets: Bonnet Carre Spillway and Morganza Spillway. These openings in the levee provide an "escape valve" for high water on the River, protecting against flooding in cities downstream, particularly New Orleans.
Geology in the study area consists largely of riverine-derived sediments, mostly silts and clays, deposited in different eras. Layers of sands store water between layers of clay which hold the water in. Some areas have natural artesian groundwater because of the pressure from recharge in northern parts of the study area. Click on the map for a larger and more detailed picture.
Aquifers and Recharge
The Chicot, Southern Hills, and New Orleans (Mississippi Alluvium) aquifer systems generally consist of alternating beds of unconsolidated and semi-consolidated sand, gravel, silt, and clay deposited in flu- vial, deltaic, and near-shore marine environments. Beds in the aquifers generally dip and thicken towards the south and southeast.
The Chicot, Evangeline, and Jasper aquifers are the principal sources of ground water to southwestern Louisiana. The Chicot aquifer is pumped for both rice irrigation and drinking water. The Southern Hills and New Orleans aquifer systems are the primary sources of drinking water in much of southeast Louisiana, but are largely pumped for industrial use. Because of their regional importance, the Chicot and Southern Hills aquifer systems are designated as sole-source aquifers by the Environmental Protection Agency.
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