Brackish or brack water is water with a level of salinity between freshwater and seawater. In many places around the world, brackish water appears naturally, and it forms an important habitat for some unique animal species. However, it can cause environmental damage, since it is harmful for organisms which have not adapted to it. This becomes an issue when brackish water is deliberately cultivated, as is done in some regions to farm desirable food fish. It is also unpleasant to drink, and it may cause health problems.

The word is derived from the Dutch brak, which means “salt.” The term was first used to describe portions of the potable water table which had been contaminated by salt water in the 1500s. The mixing of salt and fresh created mildly salty water which was not as salty as seawater, but still distasteful. Many people also noted that the brackish water was harmful, due to unique microorganisms which cause human illness that thrive in it.

In nature, estuaries are a common site of brackish water. An estuary is a location where salt and fresh water mix, typically around the opening of a river. The estuary environment is quite distinctive, as it bridges oceans and rivers, hosting unique fish, plants, and animals. When the balance of an estuary is disrupted, it can be serious for the animals which call the estuary home. Many anadramous fishes appreciate estuaries, because the slow change in salinity allows the fish to accustom themselves.

Mangrove swamps are also classically brackish. Many mangroves are located along shorelines, so there is a transitional area of saltiness which ebbs and flows with the tide. Many fish breed in mangrove swamps, and unique plants call these regions home. In some regions, mangroves are an important buffer zone between the ocean and the land, protecting the land from storm surge as a result of hurricanes and tsunamis.

Semi-salty water can also be found in some inland lakes and seas such as the Caspian Sea. The salinity in these lakes and seas may vary, depending on water flow and time of year, and they often host a range of unique animals and plants. It is not uncommon to see stratification in these bodies of water, with cold salt water on the bottom and warmer fresh or brackish water on the top. Each layer hosts different animals, and disruption of the layers can have unfortunate results for the local ecology.