U.S. Department of the interior The Pontchartrain Watershed: Fact Sheet U.S. Geological Survey A Unique Resource of the Coast

Introduction accurately described as an csluarine increased its urban acreage by 67 percent The mission of the U.S. Geological ernbayment than a lake. The lake covers from 1956 to 1988. The area along the Survey (USGS) is to provide the almost 635 square miles but is shallow northern shore of the lake also is rapidly hydrologic information and (fig. 2). ranging from 6 to 16 feet deep increasing in population. Commercial understanding needed for the best with an average depth of 11 feet. Wind is fishing remains an important contributor management of the Nation's water the major factor that controls circulation to the local economy, with revenues resources. As a part of this effort, the patterns in the lake, although small tidal estimated at S35 million annually. Fish USGS has been involved in monitoring changes also affect currents in the lake. and shellfish harvested for both the water quality of surface- and ground- A wide range of land uses, commercial commercial and recreational purposes water resources of the fisheries, and other recreational uses place include blue crab, brown shrimp, white watershed in southeastern Louisiana varied demands on the lake. The New shrimp, red drum (redlish), spotted (fig. 1) si nee 1927. Orleans, Louisiana, metropolitan area seatrout (speckled trout), catfish, and The exact origin of the lake is unknown, but it probably was formed along with Land as a series of estuarine several thousand years ago, when Bayou Sauvage was the main of the Mississippi . As the course of the shifted. Lake Pontchartrain eventually became isolated from the river. However, the lake still received periodic inflow from the river. Unusually large spring occasionally broke through the river's natural and flowed into the lake. A break in the natural was known locally as a crevasse. The present-day Bonnet CarrC Spillway is located at the site of a crevasse that last opened in 1874 Figure 2. Variation in depth of lakefloor, Lake Pontchartrain, Louisiana. and remained open for 10 years. menhaden. Lake Pontchartrain (fig. 1) retains a The Bonnet Carre" Spillway, located in connection to the through the southwestern part of the lake, is a two tidal passes, the and Chef major component of a series of Hood- Menteur Pass, and, therefore, is more control structures constructed after the disastrous Mississippi River of 1927. The structure, completed in 1932. consists of 350 concrete bays, each equipped with removable timber beams. The structure can divert as much as 250,000 ft3/s (cubic feet per second) of Mississippi River water into the lake to lower flood stages in the river in the area. The structure MILES has been opened seven limes for flood

KILOMETERS control purposes, most recently in 1983. Water Quality of the Watershed One of the primary characteristics of an is the tidal exchange of saltwater Figure 1. Lake Pontchartrain watershed, Louisiana. and freshwater. However, the tidal range in the lake is very low, only about the Amite River has an annual mean the adverse effects of 1/4 foot. Therefore, the flushing action of discharge of 2,117 ftVs. in the lake. Concerns have been the is slow, estimated to be 90 to 120 Monitoring Programs expressed by user groups that the days for water entering the western part of Water quality in Lake Pontchartrain is additional river water would promote the lake to exit through the Rigolets. monitored by the Louisiana Department algal blooms and reduce harvests by Salinities of the lake range from fresh of Environmental Quality and the fisheries. (less than 0.5 parts per thousand salt) in USGS. More than 100 sites have been Toxic contamination. In general, there late winter to brackish (about 10 parts per established to determine and evaluate the is no indication of toxic substances in thousand) in late summer. Salinities also quality of the lake, about 10 of which are Lake Pontchartrain exceeding U.S. vary from east to west and from the currently (1995) in operation. The sites Environmental Protection Agency alert surface to the bottom and are affected by also are utilized for problem-oriented limits. However, improper boat complex interactions between weather research in hydraulics, hydrology, and maintenance; automobile exhaust; and tides. Historically, seasonal salinity related fields to understand the lake stormwater runoff from highways; and changes were gradual, allowing the lake processes sufficiently well to predict their improper disposal of motor/engine oil, to be inhabited by a wide range of response to stress, either natural or household cleansers, and lawn estuarine organisms. Estuarine organisms human-induced. Water-quality maintenance products all can introduce typically require different salinities at constituents monitored in Lake toxic elements, excessive fertilizers, and different stages in their life cycles, and Pontchartrain include: nutrients such as pesticides into a lake with a limited they have adapted to enter and exit the nitrogen and phosphorus; inorganic capacity to assimilate or remove them. lake at different times of the year. There chemicals such as chloride, sodium, and Sustained monitoring and quality-control are now (1995) indications mat human- calcium; trace elements such as lead, measures are necessary, because the induced alterations in tributary flows, cadmium, and mercury; phytoplankton; cumulative effect of these point and non- , and canal construction have and pesticides and other synthetic organic point source contaminants would altered this gradual cycle, making salinity compounds. substantially reduce the ability of this changes more abrupt Many organisms, The USGS, in cooperation with the U.S. urban estuary to support a diverse plant especially juvenile fish and larval Army Corps of Engineers, has maintained and animal community. shellfish, are not well-adapted to abrupt six monitoring sites on the Mississippi Communication and change. River between St. Francisville and Coordination In addition to tidal exchange, five major Venice, Louisiana, since 1973. Four sites Communication and coordination among tributaries contribute water to Lake are currently in operation. Long-term the scientists in the USGS and members Pontchartrain: the Pearl, Tchefuncte, data from these sites will be useful in of the public and private sectors are Tangipahoa, Tickfaw, and Amite . evaluating the effects of freshwater critical components of the USGS mission. The Pearl River flows into the Rigolets diversions of river water into Lake Interagency cooperation has proved and Lake Borgne, freshening the eastern Pontchartrain and Louisiana's coastal highly effective and consists of part of Lake Pontchartrain, especially in marshes. coordinated activities by Federal, State, the spring. The annual mean discharge Issues and local agencies; universities; and the for the Pearl River near Bogalusa, . The processes by private sector that have water-resources Louisiana, is 10,070 ftVs. The amount of which nutrients are added and consumed responsibilities or interests. Specific Pearl River water entering the lake is in the lake ecosystem are known as activities of these working groups controlled by interactions between the eutrophication. If more nutrients are include: river stage and tidal action. added to a lake than the lake can Exchange of information on and The Tchefuncte and Tangipahoa Rivers assimilate, this enrichment can contribute prioritization of water-quality issues of flow directly into Lake Pontchartrain. to the growth of noxious algal blooms, regional and local interest. The has an annual mean depletion of dissolved oxygen, and fish Identification of sources of water- discharge of 163 ft3/s, and the Tangipahoa kills. The addition of excessive nutrients quality data and other information. River has an annual mean discharge of from sources such as non-point runoff Assistance in design and scope of 1,176 ft3/s. In addition to the freshwater from the northern and southern shores project elements. and nutrients needed by the lake's may increase the potential for algal Review of project planning activities, ecosystem, these rivers also add blooms and degrade the water quality for findings, and interpretations, including fertilizers, pesticides, and fecal-coliform commercial fisheries and recreational reports. bacteria from agricultural and residential uses. Dennis K. Demcheck areas; dairy farm waste; and urban Freshwater diversions. A site near the storm water runoff. Bonnet Carre Spillway has been proposed The Tickfaw and Amite Rivers also For more information, please contact: for the construction of a controlled District Chief contribute freshwater and non-point freshwater diversion of Mississippi River U.S. Geological Survey pollutants to the lake through Lake water to enhance oyster production in 3535 S. Sherwood Forest Blvd. Maurepas. The Tickfaw River has an Mississippi Sound, add sediments to Suite 120 annual mean discharge of 387 tf/s, and Baton Rouge, Louisiana 70816 benefit swamps and marshes, and reduce (504)389-0281

April 1995 U.S. Department of the Interior U.S. Geological Survey Fact Sheet FS-118-95