The Laguna Madre is the name of two long, shallow bays along the western coast of the Gulf of Mexico in the United States and Mexico; the two being separated by the outlet of the Rio Grande. Meaning “mother lagoon” in Spanish, the Laguna Madre proper is 130 miles (209 km) long, the length of Padre Island; its biological corridor, though, extends well into Mexico, to the mouth of the Río Soto la Marina in the state of Tamaulipas (see map at right).
In the United States, Laguna Madre is separated from the Gulf of Mexico on the east by Padre Island, and bounded on the west by mainland Texas, and extends from Corpus Christi in the north to Port Isabel in the south. In Mexico, Laguna Madre is separated from the Gulf of Mexico on the east by a number of barrier islands, including Barra Los Americanos, Barra Jesús María, and Barra Soto la Marina. It is bounded on the west by mainland Tamaulipas.
The Laguna Madre is very shallow, with an average depth of only 0.9 m. The lagoon is connected to the ocean by only two narrow inlets, so the tidal range – which is already minor in this part of the Gulf of Mexico – is negligible. Atmospheric effects are much more important than tides in its circulation; its weak currents generally follow the prevailing winds, and these winds can influence the water level by as much as a meter.
Oceanographically, the Laguna Madre is considered a hypersaline lagoon; this indicates that it is usually much saltier than the ocean, due to being nearly landlocked in a semiarid environment, and is one of only six hypersaline lagoons in the world. Its salinity generally increases from south to north, with distance from its major inlet near Port Isabel, but it is difficult to determine an average figure. This is because its salinity can vary wildly depending on rainfall and freshwater inflow, from as high as 120 ppt (12%) – over three times saltier than the ocean – to as low as 2 ppt (0.2%) after a heavy rain.
The Laguna Madre is one of the most important wildlife refuges on the U.S. coast, as home to many species of fish, migratory birds, sea turtles, and even wildcats. It is also one of the most important bird wintering habitats in Mexico.
Thanks to lobbying and studies done by organizations such as Pronatura Noreste, universities, local governments, and other organizations, with the aid of local communities, in April 2005 the Mexican government declared Laguna Madre and the Río Bravo’s Delta a Natural Protected Area. The 1.4 million acres (5,700 km²) under legal protection are contained in the municipalities of Matamoros, San Fernando and Soto la Marina, in the state of Tamaulipas.