On the coast, rainfall mixes with seawater from the Gulf, resulting in a coastal system called an estuary. Many of the external influences on an estuary are the same influences that affect the weather, like rainfall or winds; but the estuary is also impacted by an additional set of factors, including tides or river diversions. This daily interaction of freshwater and seawater is as complex as our local weather, and it is almost as important.
Lake Pontchartrain Basin Foundation (LPBF) sees a parallel between water monitoring and weather forecasting. Just as weather networks and websites use maps to forecast the weather, LPBF will use a map to show water movement and the most recent distribution of salinity across the basin. LPBF has developed a map to display hydrology for the Pontchartrain Basin. We call our map the “Hydrocoast Map.” Please see below to view LPBF's latest Hydrocoast Maps. You can also view our archived maps and methodology.
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Click on link below map for larger image.
The Hydrocoast maps for the week of January 25 through January 31, 2016 were produced using field data, MODIS, satellite -imagery, precipitation data, wind data and permanent monitoring stations in the basin (USGS buoys, Coastwide Reference Monitoring System (CRMS), etc. During this Hydrocoast period Lake Pontchartrain was fresh due to the opening of the Bonnet Carre Spillway with the entire lake being under 3 ppt. The entire basin has freshened due to Mississippi River flood, Bonnet Carre Spillway opening and rainfall. A steep salinity gradient developed in the sounds because of the fresh water flowing out and salt water pushed in by southeast winds. Discharge increased at all Northshore rivers, Bonnet Carre Spillway, Fort St. Philip, Main Pass, Pass a Loutre and Southwest Pass and decreased at all other outlets. Average daily discharge difference between last Hydrocoast and the current was:
- Pearl River = 7,311 to 16,664 cfs
- Tangipahoa = 971 to 1,876 cfs
- Tickfaw = 422 to 1,040 cfs
- Amite = 2,964 to 4,376 cfs
- Caernarvon Diversion = 111 to 36 cfs
- Mardi Gras Pass = 9,100 to 8,600 cfs
- Violet Siphon = Closed
- Bohemia Spillway = 41,174 to 33,806 cfs
- Bonnet Carré Spillway = 78,000 to 95,000 cfs
- Bayou Lamoque = 3,513 to 3,056 cfs
- Fort St. Philip = 59,981 to 165,000 cfs
- Baptiste Collete = 117,749 to 96,382 cfs
- Grand and Tiger Pass = 108,257 to 102,233 cfs
- Main Pass = 96,521 to 127,386 cfs
- West Bay = 76,028 to 58,000 cfs
- Pass A Loutre = 74,151 to 100,779 cfs
- Southwest Pass = 333,561 to 381,894 cfs
- South Pass = 149,236 to 95,755 cfs
The first map (Hydrocoast map without precipitation) shows the salinity contours and freshwater discharge across the Pontchartrain Basin. The solid line salinity contours are at 1 ppt salinity increments. The salinity is highest out past the Chandeleur Islands (red lines, 32 ppt) and decreases to fresh conditions (dark blue lines) in the basin. Sea water generally has a salinity of 32 ppt. Green asterisks represent salinity leak points, usually in the form of gates in storm surge protection features but also in passes, portions of roads that are raised, canals, and bayous that are holes in the salinity barriers (pink lines) in the form of levees, roads, natural ridges and canals (with associated spoil banks). Salinity contours that are close together represent an area where salinity changes quickly over a short distance, which is seen slightly offshore throughout the basin during this Hydrocoast period. Contours that are farther apart represent a more gradual change over longer distances which can be seen in throughout the interior of the basin, in Lake Pontchartrain and Lake Maurepas.
The second map shows the salinity contours laid on top of a habitat/land -use map, showing where fresh to salt marsh and swamps are found in the basin. This map also shows soil water salinity contours across the land masses. The third map shows the salinity contours in conjunction with weekly rainfall and wind roses in the Pontchartrain basin. During this Hydrocoast period there was rain across the basin ranging from 1 to 3 inches. Resultant winds were from the southeast. Wind speeds ranged from 2 to 11.2 m/s (2 to 25 miles/hr).
The fourth "Water Quality" shows the results of LPBF's water quality sampling around Lake Pontchartrain, reporting the fecal coliform counts. There were elevated fecal coliform counts at Abita River and Liberty Bayou. There were high fecal coliform counts at Bogue Falaya, Tchefuncte River, Little Tchefuncte River, Cane Bayou, Ponchatoula Creek/LA22, Yellow River/LA22, Natalbany River/LA22, and Tickfaw River/LA22. The water quality map also shows the impaired water bodies for Primary Contact (swimming, immersion likely) and Secondary Contact (boating wading, immersion unlikely) in the basin as prepared by the Louisiana Department of Environmental Quality under the EPA 305(b)/303(d) guidelines.
The fifth “Biological Map” shows oyster harvest area closure, as determined by the Louisiana Department of Health and Hospitals and the location of the public oyster seed grounds. During this Hydrocoast period, oyster harvest areas 1-8 were closed due to the threat of red tide. The biological map also shows the impaired water bodies for fishing and oyster propagation in the basin as prepared by the Louisiana Department of Environmental Quality under the EPA 305(b)/303(d) guidelines. Impairments due to metal contamination and fecal coliform are shown.