Brackish water

Some water is a mix. It has more salt than river water. But it has less salt than the sea.

Many fish use these areas. They use them as nurseries. Young fish can eat and grow there. Some fish travel through to reach other homes.

Swamps can be brackish too. Trees called mangroves grow there. Some fish in these swamps spit at bugs. This helps them catch food.

Some big lakes and seas are brackish. The Caspian Sea is the largest lake. It has special seals that live there. It is a very interesting place.

Brackish water is a mix of water types. It has more salt than fresh river water. But it has less salt than the sea.

This often happens in estuaries. An estuary is where a river meets the sea. The River Thames in London is a famous estuary. In some parts, the water is fresh. In other parts, it becomes brackish. Near the sea, the water is fully salty. Many fish live here. Some fish, like salmon, travel from the sea to rivers. These fish use estuaries to adjust to the salt. Other fish use these areas as nursery grounds. This means young fish can eat and grow safely.

Brackish water is also found in mangrove swamps. These are forests that grow near the sea. Many animals live in these swamps. You might see crocodiles or monkeys there. Some fish, like the archerfish, live in these trees. They spit at bugs to catch them.

Some large lakes are also brackish. The Caspian Sea is the largest lake in the world. It has special seals that live in its water. The Baltic Sea is another example. In that sea, fresh water stays on top. Salt water stays at the bottom.

Caption: A fish that lives in brackish water.

Brackish water is a special kind of water found in nature. It is not quite fresh like a river, but it is not as salty as the ocean. This happens when fresh water and salt water mix together. It can also be found in old underground water sources called fossil aquifers. The word comes from a Middle Dutch root called "brak." This water is important because it creates unique homes for many living things.

This water works by changing its salt levels in different places. Scientists measure this using parts per thousand. Brackish water usually has between 0.5 and 30 grams of salt in every litre. If the salt goes above 30 parts per thousand, it is called saline water. In some places, like the Baltic Sea, the water layers itself. The heavy salt water sinks to the bottom. The lighter fresh water stays on the top.

Many famous places show us how this works. The River Thames in London is a great example of an estuary. An estuary is where a river meets the sea. Near Battersea, the Thames starts to become brackish. Further east, near Gravesend, the water becomes fully marine. This change in salt helps different fish move through the river. Some fish, like salmon, live in the sea but swim up rivers to spawn. Others, like eels, live in rivers but go to the sea to breed.

There are also amazing habitats like mangrove swamps. These are forests that grow where the salt levels change with the tides. Many animals live here, such as the saltwater crocodile and the proboscis monkey. You can even find archerfish that spit at insects to eat them. These swamps act as a buffer zone for the land. They can help protect the coast from damage caused by tsunamis or hurricanes. Large forests like the Sundarbans are found in the Bay of Bengal.

Humans also interact with brackish water in many ways. We use it for cooling power plants or in the oil and gas industries. Some people even make brackish water pools to farm freshwater prawns. We can turn this water into fresh water using a process called reverse osmosis. This is a type of filtration that cleans the water. Large bodies like the Caspian Sea also hold brackish water. The Caspian Sea is the largest lake in the world and is home to the Caspian seal.

Brackish water describes a specific type of environment where salinity levels fall between those of freshwater and seawater. It is not a precisely defined condition because the salt concentration can change across different spaces and times. Technically, brackish water contains between 0.5 and 30 grams of salt per litre. This is often expressed as 0.5 to 30 parts per thousand (‰). The specific gravity of this water typically ranges from 1.0004 to 1.0226. If the concentration rises above 30‰, the water is classified as saline. This unique chemistry creates specialized habitats that support distinct biological communities.

One common way brackish water forms is through the mixing of fresh and salt water. This occurs most frequently in estuaries, which are areas where a river meets the sea. In these zones, the salinity levels shift as you move toward the ocean. For example, the River Thames in London demonstrates this transition clearly. Near Battersea, the Thames becomes brackish, and the variety of freshwater fish begins to decrease. By the time the river reaches Gravesend, the conditions become fully marine. This process creates an ecological succession where different species replace one another based on their salt tolerance.

Many animals have evolved to survive these changing conditions. Euryhaline species are organisms that can tolerate a wide range of salinity. In the Thames, marine species like flounder, European seabass, mullet, and smelt become common as salinity rises. Estuaries also serve as vital staging points for migratory fish. Anadromous fish, such as salmon and shad, live in the sea but swim up rivers to spawn. Conversely, catadromous fish, like eels, live in rivers but return to the sea to breed. These habitats also act as nursery grounds where young fish, including herring and plaice, can feed and grow safely.

Another critical brackish habitat is the mangrove swamp, also known as a mangal. These forests often fringe estuaries and lagoons where tides constantly change the salt levels. Mangroves are home to highly specialized residents like mudskippers, which forage on land. Archerfish also live here, spitting at insects to knock them into the water. These swamps provide breeding grounds for snappers, halfbeaks, and tarpon. Beyond fish, animals like the saltwater crocodile and the proboscis monkey rely on these areas. Mangroves also serve as important nesting sites for many bird groups, including herons and kingfishers.

Beyond being biological hubs, these environments serve important physical functions. Mangrove swamps act as buffer zones between the land and the sea. They provide a natural defense against damage from hurricanes and tsunamis. Some large mangrove forests, such as the Sundarbans and Bhitarkanika in the Bay of Bengal, are massive ecosystems. In larger bodies of water, salinity can even cause the water to layer itself. In the Baltic Sea, the denser seawater stays at the bottom while freshwater stays on top. This stratification means cod live in the deep marine layers, while pike stay in the surface waters.

Brackish water is also found in unique geographical locations like the Caspian Sea. As the world's largest lake, the Caspian contains salinity about one-third that of normal seawater. This supports peculiar fauna, including the Caspian seal and the great sturgeon. Hudson Bay is another example, staying brackish due to limited ocean connections and high freshwater runoff. Even inland reservoirs can become brackish, such as Lake Texoma on the U.S. border of Texas and Oklahoma. This lake became brackish because the Red River carries salt from buried deposits upstream. This allows striped bass, a saltwater fish, to live there.

Human activity both creates and utilizes brackish water. Civil engineering projects, such as building dikes, can produce these conditions. Humans also flood coastal marshlands to create brackish pools for farming freshwater prawns. In industry, brackish water is used for cooling power plants and in the oil and gas sectors. It can also be processed for use in agriculture or municipal supplies. Through methods like reverse osmosis or electrodialysis, humans can filter the salt out. This makes the water usable for drinking or livestock, turning a challenging environment into a valuable resource.