Hydrology


landforms Image and information from the Coast 2050 report

The major landforms of the Louisiana Coastal wetlands consist of the Deltaic Plain and the Chenier Plain.  All of the landforms are from deltaic formation as described in the wetland formation section.  The sources of sediment include many rivers and lake outlets, which form into a network of distributaries once they reach the wetlands.  The wetlands are fed by the Mississippi, Sabine, and Atchafalaya rivers, Lake Pontchartrain, Calcasleu Lake, Wax Lake, Teche Bayou, and Lafource bayou.  Water from these surface waters is trapped in the wetlands due to a combination of vegetation slowing flow and a thick layer of semi-impermeable river mud/clay underlying the coastal wetlands.

The Louisiana Coastal wetlands are far from being flat; countless ridges run through them, providing crucial upland habitat.  These ridges were formed as ancient barrier islands were incorporated into the marsh.

Historically, groundwater may have been an important water source feeding the wetlands, but drainage networks, canals, and groundwater withdrawals have both reduced this input, resulting in saltwater intrusion into drinking water supplies.




Mississippi River
The Mississippi River is the largest river system in North America, with an approximate discharge of 15,400 cubic meters/second.  Annual sediment discharge is approximately 240 billion kilograms and is primarily composed of clay with silt and sand.  
Deltas form as the flows break over natural levees and begin to form distributaries as shown on the right.
Flow of the Mississippi is seasonal, peaking in late spring due to snow melting in the upper watershed.  River flow is almost independent from local rainfall due to the immense watershed.  


Tidal Influence

Tidal flows are another important aspect of the hydrology of the coastal wetlands.  High tides send salt water far into the marshes leading to inundation, which replenishes nutrients, removes wastes, and increases subsidece.  The Diurnal tides average 30 centimeters.  Tides in the region show seasonal variation due to dominant wind direction.  In the fall, winds from the the East and Southeast cause a  sea level rise of up to 20 centimeters.  Summer and winter Westerly winds draw water levels down.  Larger tidal variations exist due to extreme weather and can send salt water far inland.  Strong storms can result in tidal increases of up to 3.5 meters.  Considering an average slope of  only 0.2 centimeters per kilometer, extreme tides can inundate vast expanses of wetland.  

  

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