Laguna Madre near Gulf Coast, Mexico25.0N 97.6W
Near the top of this image is the border between Texas, USA (above) and Mexico (below). Parallel to the coast is the Laguna Madre, 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. The lagoon is bounded on the west by mainland Tamaulipas. It is located in the municipalities of Matamoros, San Fernando, and Soto la Marina.
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. 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.