Brackish water is a combination of salt water and fresh water that usually occurs in estuaries, where freshwater rivers empty into the sea. Some species of plants can tolerate brackish water, but many cannot. When brackish water is introduced to an area that originally contains freshwater, usually as a result of human activity, many species of plants can be killed. This is a major concern in the wetlands of Southern Louisiana. There are also lakes and seas which are naturally made of brackish water.

The water in Lake Pontchartrain is brackish.