What Is Brackish Water

Short answer

• Brackish water is water that is saltier than fresh water but less salty than seawater. It forms where fresh water and salt water mix, most often where a river meets the sea, and also in coastal lagoons, mangrove wetlands, some inland lakes and seas, and some underground aquifers. There is no single salinity number that every authority uses for brackish water, and that is the first thing to understand. General references commonly cite 0.5 to 30 parts per thousand of salt for brackish surface water, with fresh water below 0.5 parts per thousand and seawater at about 35 parts per thousand, but no single institutional source checked for this article confirms 30 parts per thousand as a settled upper limit, and other classifications use different upper limits, so treat any single upper limit as a convention, not a universal boundary. The United States Geological Survey, for its national brackish groundwater assessment, used a different working definition: dissolved solids from 1,000 to 10,000 milligrams per liter, with seawater at 35,000 milligrams per liter. The two frames measure related but different things, so this article attributes every salinity figure to its frame instead of presenting one range as universal. Brackish water matters because it is not just diluted seawater. It is a shifting habitat with its own plants and animals, farming and fishing uses, and problems, from salt water creeping into wells to pollution trapped in slow moving estuaries.

What Brackish Water Means

The word brackish describes water in the middle of the salinity scale. Fresh water, such as most river and lake water, tastes fresh because its dissolved salt content is low. Seawater tastes strongly salty because its dissolved salt content is high. Brackish water sits between the two: salty enough to taste, and salty enough to matter for plants, animals, pipes, and crops, but not as salty as the open ocean.

Salinity is the measure of dissolved salts in water. It is usually reported in parts per thousand, written ppt, which means grams of salt per kilogram of water. It can also be reported as milligrams per liter, written mg/L, which is the unit the United States Geological Survey uses for dissolved solids in groundwater. Total dissolved solids, often shortened to TDS, is a related measure that counts all dissolved minerals, not only table salt. Sodium chloride is the dominant salt in seawater, but brackish water also contains other dissolved minerals, in proportions that vary with the source of the water.

A sample taken from a river mouth at high tide can be much saltier than one taken from the same spot at low tide or after heavy rain, and a coastal aquifer can be brackish for a different reason: ancient seawater trapped underground, or seawater drawn inland by pumping. The label brackish therefore describes a range and a condition, not a fixed recipe.

At the fresh end, water with less than 0.5 parts per thousand of salt is generally called fresh water. At the salty end, average seawater sits at about 35 parts per thousand. Between them lies the brackish band. Under the commonly cited general convention, that band runs from 0.5 to 30 parts per thousand. Water above that cited band is usually classed as saline in that general classification rather than brackish, even though it may still be slightly less salty than average seawater. This convention suits estuaries, lagoons, and coastal waters, where salinity changes with tides and river flow.

The groundwater picture uses different numbers because it measures a different thing. The United States Geological Survey states that quantitative definitions of brackish vary, and that brackish groundwater is generally understood to have more dissolved solids than fresh water but less than seawater. For its national assessment, the Survey defined brackish groundwater as water with dissolved solids from 1,000 to 10,000 milligrams per liter, and it put seawater at 35,000 milligrams per liter. The Survey also notes that the term saline commonly refers to any water with more than 1,000 milligrams per liter of dissolved solids, which means the saline label overlaps the brackish range in groundwater usage. Readers who compare a groundwater report with an estuary report should therefore check which definition each report uses before comparing the numbers.

Local authorities can differ again. The Chesapeake Bay Program describes most of the water in the Chesapeake Bay, including its middle portion and its tidal rivers, as brackish with a salinity greater than 0.5 parts per thousand but less than 25 parts per thousand. That upper limit is lower than the 30 parts per thousand in the general convention because it describes the water actually found in that bay. Always ask which water, which measure, and which authority stands behind a salinity figure.

Why There Is No Single Fixed Definition

Brackish water resists a single tidy definition for three reasons. First, the sources of salt differ. In an estuary, the salt comes mainly from the sea. In a brackish aquifer, it can come from ancient seawater, from minerals dissolved out of rock, or from modern seawater intrusion. In an inland brackish lake, it can come from inflowing rivers that carry dissolved minerals into a basin with no outlet, where evaporation then concentrates the salts left behind. Each source produces a different mineral mix, so two waters with the same total salinity can behave differently.

Second, brackish water moves. Tides push seawater upstream in many estuaries, then let fresh water push back. Seasons change the balance: heavy rain and snowmelt increase fresh water flow and push the brackish zone toward the sea, while dry seasons reduce river flow and let salt water reach farther upstream. Wind can pile water against a coast or push it offshore, shifting the mixing zone in a shallow bay. A single salinity reading is therefore a snapshot, not a permanent property of a place.

Third, brackish water is layered as well as mixed. Salt water is denser than fresh water, so in many estuaries the heavier salt water slides underneath the lighter fresh water, forming what is called a salt wedge. Near the surface, the water may be nearly fresh. Near the bottom, it may be nearly as salty as the sea. Between the layers there can be a sharp transition zone, called a halocline, where salinity changes rapidly with depth. In a well mixed estuary with strong tides, by contrast, the layers are stirred together and salinity changes more gradually from river to sea. Both patterns are brackish, but they create very different conditions for the organisms living in them.

How Brackish Water Forms

Brackish water forms whenever fresh water and salt water meet and mix, or whenever fresh water picks up enough dissolved salts to leave the fresh range without reaching seawater strength. The most common route is mixing at the coast, but there are several others.

The classic route is the estuary. A river carries fresh water toward the sea. Tides carry seawater into the river mouth. The two waters meet, mix, and produce a brackish zone that stretches upstream as far as the tide can push salt water, and downstream as far as river water can dilute the sea. The National Oceanic and Atmospheric Administration describes estuaries as bodies of water usually found where rivers meet the sea, home to plant and animal communities adapted to brackish water, a mixture of fresh water draining from the land and salty seawater. That definition captures the essential point: an estuary is defined by mixing, not by a single salinity value.

Deltas form brackish water in a similar way, but across a fan of channels rather than a single river mouth. Where a large river splits into many distributaries before reaching the sea, each channel mixes fresh and salt water, and the marshes and shallow bays between channels become brackish as well. The balance shifts with each flood and each dry spell, so delta wetlands can swing between nearly fresh and nearly marine conditions over a season.

Coastal lagoons and bays become brackish when a barrier island, sandbar, or reef partly closes off a shallow coastal water body from the open sea. Fresh water enters from rivers and rainfall, salt water enters through inlets during tides and storms, and the restricted exchange keeps the mixture in the brackish range. If an inlet closes, the lagoon may freshen. If a storm breaches the barrier, it may turn saltier. Lagoons are therefore among the most changeable brackish waters.

Mangrove wetlands are a special case of coastal mixing. Mangroves grow along tropical and subtropical shores where rivers meet the sea, and their roots stand in water regularly flooded by tides. The water around them is brackish for much of the tidal cycle, though it can approach seawater strength at high tide in the dry season and freshen after heavy rain.

Inland, brackish water forms without any direct connection to the sea. Some lakes sit in closed basins where rivers flow in but no river flows out, and evaporation leaves dissolved minerals behind, so the lake gradually becomes saltier than the rivers feeding it. Groundwater can also turn brackish underground, where fresh water meets ancient saline water, or where heavy pumping draws seawater inland into wells that were once fresh. Irrigation drainage can create brackish water too, by carrying salts leached from soil back into rivers.

Where Brackish Water Occurs

Brackish water occurs wherever fresh water and salt water meet or where dissolved salts accumulate in a basin. Estuaries are the largest and most ecologically important brackish habitats. They include river mouths, tidal creeks, coastal bays, and the tidal reaches of rivers far upstream from the coast. The Chesapeake Bay, on the east coast of the United States, is the largest estuary in the United States and one of the largest in the world, according to the Chesapeake Bay Program, and most of its water is brackish. In each case, the brackish zone moves with the tide and the season.

Deltas extend the estuarine pattern across a broad, flat landscape. The Mississippi Delta, the Nile Delta, and the Ganges-Brahmaputra Delta all contain vast brackish marshes, channels, and shallow bays, where people farm, fish, and live, so the brackish balance is shaped by levees, diversions, and drainage as well as by tides and floods.

Mangrove coasts fringe much of the tropical world, from Florida and the Caribbean to Southeast Asia, northern Australia, and East Africa. The water among mangrove roots is brackish to saline depending on tide, rainfall, and position along the shore. Mangroves are listed by the National Oceanic and Atmospheric Administration among common estuarine habitats, alongside oyster reefs, coastal marshes, submerged aquatic vegetation, and riparian forests. Their presence is a practical field clue: where mangroves thrive, the water is rarely purely fresh or purely marine.

Coastal lagoons and barrier island bays add another set of brackish waters, often shallower and warmer than open estuaries, including the lagoons behind barrier islands along the Atlantic and Gulf coasts of the United States. Because they are shallow, they respond quickly to storms, so their salinity can swing sharply after a single weather event.

Brackish lakes and seas show that brackish water is not only a coastal phenomenon. The Baltic Sea, in northern Europe, is often described as brackish because heavy river inflow and limited exchange with the North Sea keep much of it well below seawater salinity, with fresher water near the surface and saltier water at depth. The Caspian Sea, the largest enclosed inland water body on Earth, is also broadly brackish, though its salinity varies from fresher in the north, where the Volga River flows in, to saltier in the south. These inland examples are a reminder that brackish is a salinity condition, not a location: wherever the salt balance lands between fresh and marine, the water is brackish, whether or not the sea is nearby.

Finally, brackish groundwater lies beneath many coastal plains and some inland basins. The United States Geological Survey undertook a national assessment of brackish aquifers precisely because this hidden resource is widespread in the United States and could supplement fresh water supplies in water stressed regions. Unlike an estuary, a brackish aquifer does not change with the tide, but its salinity can change over years as pumping, recharge, and sea level shift the balance underground.

Why Brackish Water Changes Through the Day and the Year

Tides are the fastest driver. In a tidal estuary, seawater advances upstream on the rising tide and retreats on the falling tide, so salinity at a fixed point rises and falls with the tidal cycle. Organisms that cannot move, such as oysters and marsh plants, must tolerate the full swing. Mobile animals, such as fish and crabs, often move with the tide, following water of a salinity they prefer, a behavior the National Oceanic and Atmospheric Administration notes when it describes estuarine animals adjusting to changing conditions through the day.

River flow is the seasonal driver. In spring, snowmelt and rain swell rivers and push the brackish zone seaward. In late summer or during drought, river flow falls and seawater intrudes farther upstream, sometimes reaching water intakes and farms that are fresh for most of the year. In monsoon climates, the swing can be extreme: a delta that is nearly fresh in the wet season can be strongly brackish in the dry season.

Evaporation matters most in shallow lagoons, salt marshes, and inland basins: hot, dry weather leaves salts behind and raises salinity, while rain dilutes it again. Wind adds a shorter term push, driving seawater into a bay during a storm. Human management overlays all of these drivers: dams change the timing of fresh water delivery, levees change where tides can reach, pumping lets salt water move inland, and irrigation drainage returns salty water to rivers.

Life in Brackish Water

Brackish water supports a distinctive community of plants and animals, not simply a diluted version of freshwater or marine life. The defining challenge is change itself: salinity rises and falls, sometimes within hours, and organisms must tolerate that swing, avoid it, or move with it.

The National Oceanic and Atmospheric Administration describes estuaries as among the most productive ecosystems in the world, supporting food, breeding places, and migration stopovers for many animals. That productivity comes from nutrients delivered by both the river and the sea and from marsh plants and algae that feed a rich food web. Oyster reefs, seagrass beds, salt marshes, and mangrove forests shelter young fish and shellfish, filter water, and stabilize sediment, and many commercially important fish and shellfish spend part of their life cycle in estuaries as a nursery before moving to the open sea as adults.

Plants in brackish water show the adaptation problem clearly. Most land plants cannot tolerate salt, because salt in the soil draws water out of roots. Marsh grasses, saltmarsh plants, and mangroves cope in different ways: some exclude salt at their roots, some excrete it through glands on their leaves, and some store it in leaves that are later shed. The result is a zoned landscape: fresher marsh plants upstream and salt tolerant species downstream.

Animals cope with changing salinity through physiology and behavior, and species that tolerate a wide salinity range are called euryhaline. Many estuarine fish, crabs, shrimps, and mollusks are euryhaline, adjusting the salt balance in their bodies as the surrounding water changes. Some fish, such as salmon and eels, go further: they migrate between fresh water and seawater during their life cycle and remodel their salt handling organs for each stage. Others avoid the problem by moving, following the tide or the season to stay within a preferred salinity band. Still others, such as oysters, close their shells and wait out an unfavorable swing.

That tolerance has limits, and the limits shape where species live. Freshwater species generally fade out as salinity rises, and fully marine species fade out as it falls, leaving a middle community with fewer species than either neighbor but often very abundant in individuals: dense beds of shellfish, clouds of small fish, and flocks of wading birds. For birds, brackish wetlands are also critical stopovers on migration.

Human Uses of Brackish Water

People have used brackish water for as long as they have lived beside estuaries, and modern uses are wider than most readers expect. Fishing and shellfish farming are the oldest large scale uses. Estuaries and brackish lagoons are fished for finfish, crabs, shrimps, and oysters, and brackish ponds are used for aquaculture in many parts of Asia and elsewhere. Brackish aquaculture suits species that naturally live in estuaries, but it needs careful management: ponds drained carelessly can release salty, nutrient rich water into nearby fresh water or farmland, and poorly sited farms can damage mangroves that shelter wild fish.

Farming near brackish water is a balancing act. Some crops tolerate mild salinity, but most staple crops suffer when irrigation water is too salty: yields fall and soils accumulate salt. Farmers in deltas therefore time irrigation to periods when river water is freshest and rely on embankments and sluices to keep salt water out of fields during dry seasons.

Drinking water from brackish sources is possible, but only after treatment. The United States Environmental Protection Agency sets a secondary standard for total dissolved solids in drinking water at 500 milligrams per liter. Secondary standards are non enforceable federal guidelines for aesthetic effects such as taste, staining, and deposits, rather than health based limits, and states may adopt them as enforceable standards. Brackish water, by either the groundwater definition of 1,000 to 10,000 milligrams per liter or the commonly cited general convention of 0.5 to 30 parts per thousand, sits at or above that 500 milligrams per liter guideline, and for most of its range well above it, so it tastes salty and is not suitable as untreated drinking water. Treatment, usually by reverse osmosis, which pushes water through a membrane that blocks dissolved salts, can bring it within drinking water quality. Because brackish water is less salty than seawater, treating it generally takes less energy than seawater desalination, which is why the United States Geological Survey assessment framed brackish groundwater as a possible supplement to fresh water supplies in water stressed regions.

Industry also uses brackish water: coastal power stations and industrial plants often use it for cooling where fresh water is scarce, and it feeds some desalination plants serving coastal towns, whose concentrated brine must be disposed of carefully. Estuaries and brackish lagoons also support boating, birdwatching, and sport fishing.

Challenges and Risks

Brackish water brings a set of problems that follow directly from its position between fresh and salt. The most widespread is saltwater intrusion into fresh water supplies. When coastal groundwater is pumped faster than it is recharged, or when sea level rises, the boundary between fresh groundwater and seawater moves inland, and wells that once produced fresh water begin to produce brackish water. Once an aquifer is intruded, recovery can take years, because the salt must be flushed out by fresh recharge. The same process affects surface water: as sea level rises and river flows change, the brackish zone in an estuary moves upstream, threatening drinking water intakes, irrigation diversions, and freshwater wetlands that were previously beyond tidal reach.

Pollution behaves differently in brackish water than in a fast flowing river or the open sea. Estuaries trap sediment and nutrients because water slows where river meets tide, and pollutants from farms, towns, and industry can accumulate in mud and in shellfish. Excess nutrients can fuel algal blooms that deplete oxygen when the algae die and decay, creating dead zones where fish and shellfish cannot survive. Because estuaries are also nurseries, pollution there affects fish populations well beyond the estuary itself.

Habitat loss compounds the problem. Mangroves are cleared for aquaculture ponds, housing, and ports. Salt marshes are drained or filled for development. Dams and levees cut estuaries off from the river sediment and fresh water pulses they need to maintain their shape. When the habitat goes, the services go with it: less nursery ground for fish, less buffering against storm surge, and less filtering of runoff.

Climate change sharpens each of these pressures. Rising seas push brackish water farther inland, changing rainfall alters river flows outside the range local species and farms are used to, and warmer water holds less oxygen. The risk comes from the speed of change and from the loss of room for habitats to move, as coastal development blocks the inland migration of marshes and mangroves. Management therefore focuses on keeping room for the gradient to move: protecting mangrove and marsh buffers, restoring tidal flow to diked wetlands, setting pumping limits near coasts, and treating desalination brine and aquaculture effluent before discharge.

Frequently Asked Questions

Is brackish water fresh water or salt water?

Neither, exactly. Brackish water is a mixture of fresh water and salt water, saltier than fresh water and less salty than seawater. Under the commonly cited general convention it runs from 0.5 to 30 parts per thousand of salt, compared with less than 0.5 parts per thousand for fresh water and about 35 parts per thousand for seawater. Groundwater reports use a different frame, from 1,000 to 10,000 milligrams per liter of dissolved solids in the United States Geological Survey assessment, so always check which definition a report uses.

Can you drink brackish water?

Not untreated. Brackish water tastes salty and sits at or above the United States Environmental Protection Agency secondary guideline of 500 milligrams per liter, and for most of its range well above it for total dissolved solids in drinking water. It can be made drinkable by desalination, most often reverse osmosis, which removes dissolved salts through a membrane, though the concentrated brine left behind must be disposed of without harming nearby water.

Where is brackish water found?

Most often in estuaries where rivers meet the sea, and in deltas, coastal lagoons, mangrove wetlands, and tidal rivers. It also occurs in brackish lakes and seas, such as the Baltic Sea and the Caspian Sea, and underground in brackish aquifers. Anywhere fresh water and salt water mix, or where dissolved salts accumulate without reaching seawater strength, the water is brackish.

What animals live in brackish water?

Estuaries support oysters, crabs, shrimps, many fish species, and wading and migratory birds that use brackish wetlands as feeding and stopover grounds. Many estuarine species are euryhaline, meaning they tolerate a wide range of salinity, and some fish migrate between fresh water and seawater during their lives.

Is the Chesapeake Bay brackish?

Mostly, yes. The Chesapeake Bay Program describes most of the water in the Bay, including its middle portion and its tidal rivers, as brackish, with salinity greater than 0.5 parts per thousand but less than 25 parts per thousand. The head of the Bay is fresh and the mouth approaches seawater strength, so the Bay as a whole is a salinity gradient.

What is the difference between brackish water and saline water?

The terms overlap, and the distinction depends on the definition in use. In the commonly cited general convention, brackish water runs from 0.5 to 30 parts per thousand, and water above that cited band is usually classed as saline in that general classification. In groundwater usage, the United States Geological Survey notes that saline commonly means any water with more than 1,000 milligrams per liter of dissolved solids, which includes the brackish groundwater range. Treat saline as the broader term and brackish as the middle band within it, and check the definition behind any particular figure.

Is brackish water safe for plants?

Most garden and crop plants are not safe in strongly brackish water: salt draws water out of roots and accumulates in soil. Marsh plants, mangroves, and seagrasses are adapted to brackish conditions, but most staple crops need fresh water or only mildly brackish water, with drainage to flush accumulated salt.

Sources & references

• United States Geological Survey (usgs.gov): Brackish Groundwater Assessment: brackish groundwater defined for the national assessment as dissolved solids from 1,000 to 10,000 milligrams per liter, seawater at 35,000 milligrams per liter, quantitative definitions vary, and the term saline commonly covers water above 1,000 milligrams per liter.
• National Oceanic and Atmospheric Administration, National Ocean Service (oceanservice.noaa.gov): What Is an Estuary: estuaries usually found where rivers meet the sea, home to communities adapted to brackish water, a mixture of fresh water draining from the land and salty seawater, and among the most productive ecosystems in the world.
• National Oceanic and Atmospheric Administration, National Ocean Service (oceanservice.noaa.gov): Estuaries Tutorial and Life in an Estuary: mangroves, oyster reefs, coastal marshes, submerged aquatic vegetation, and riparian forests as common estuarine habitats, and estuarine animals adjusting to changing salinity and tides.
• Chesapeake Bay Program (chesapeakebay.net): Bay Facts: most of the water in the Chesapeake Bay, including its middle portion and its tidal rivers, is brackish with salinity greater than 0.5 parts per thousand but less than 25 parts per thousand, and the Bay is the largest estuary in the United States.
• United States Environmental Protection Agency (epa.gov): National Secondary Drinking Water Regulations and Secondary Drinking Water Standards guidance: total dissolved solids secondary standard of 500 milligrams per liter, a non enforceable federal guideline for aesthetic effects, with states able to adopt it as enforceable.