Short answer
- A body of water is a significant accumulation of water on the surface of Earth. The term most often brings oceans, seas, and lakes to mind, but it also covers smaller and moving waters, including ponds, wetlands, rivers, and streams. A body of water does not have to be still or fully enclosed. Flowing water counts, and so do many human made waters, such as reservoirs and canals. The simplest way to sort bodies of water is to ask five questions about each one: Is it still or flowing? Is it salty, fresh, or in between? Is it natural or human made? Is it large or small? And is it connected to other waters, or closed off on its own? No single question gives a complete answer, because a large salty sea, a small fresh pond, and a flowing river differ on several measures at once. This article works through each major type in turn and shows how water moves between them.
Why One Simple Phrase Covers So Much
People use the phrase “body of water” as if it named one kind of thing. In practice it is an umbrella term. It covers water that is deep and water that is shallow, water that moves fast and water that barely moves at all, water you can drink and water that would make you thirstier if you drank it. That breadth is useful, because it lets us talk about Earth’s surface water as a whole. It is also the reason the phrase can confuse. Two features can both be bodies of water and still have almost nothing else in common.
Geographers and water scientists therefore rarely stop at the umbrella term. They describe a feature by what it is and how it behaves. Is the water contained in a basin, or does it flow in a channel? Does water flow in, flow out, or both? Is the feature fed mainly by rain, by groundwater, by melting ice, or by a larger water body? Answers to those questions matter more than the label alone. This article is a survey of the types. It does not repeat a full account of the oceans, and it does not retell the full water cycle. Those subjects have their own articles. Here, oceans appear only as one type among many, and the movement of water appears only as far as it explains how the types connect.
How Much of Earth’s Water Is Where
Before sorting the types, it helps to know how unevenly Earth’s water is shared out. The ocean covers more than 70 percent of the surface of the planet, according to the National Oceanic and Atmospheric Administration (NOAA), and it holds about 97 percent of Earth’s water. The United States Geological Survey (USGS) gives a similar picture: about 71 percent of Earth’s surface is water covered, and the oceans hold about 96.5 percent of all Earth’s water. The small difference reflects different estimates and rounding, not a disagreement about the main point. Almost all Earth’s water is salty ocean water.
Freshwater is the small remainder, about 2.5 to 3 percent of all Earth’s water, depending on the estimate used. Most of that freshwater is not in lakes and rivers at all. It is locked up in glaciers and ice caps, or held underground as groundwater. The USGS notes that freshwater lakes and swamps account for only 0.29 percent of Earth’s freshwater, and that rivers hold only about 0.006 percent of total freshwater reserves. Rivers look large on a map and matter greatly to people, but as a store of water they are tiny. Their importance comes from movement: rivers keep delivering fresh water to new places, rather than storing a large amount in one place.
Five Questions That Sort Any Body of Water
There is no single official sorting system. The five questions below are a practical way to place any named feature, and they explain most of the differences people ask about.
First, is the water still or flowing? Lakes, ponds, seas, and oceans mainly hold water in place, although currents, tides, and wind move water within them. Rivers and streams mainly move water from one place to another. Wetlands sit in between: water may be still, slow moving, or present only seasonally.
Second, is the water salty, fresh, or brackish? Freshwater, as defined by the USGS, contains less than 1,000 milligrams per liter of dissolved solids, most often salt. Seawater is far saltier. Brackish water sits between fresh and salty, and it is typical of estuaries, where river water meets the sea.
Third, is the feature natural or human made? Most oceans, seas, lakes, and rivers are natural. Reservoirs and canals are human made. Some features can be either: a lake can be natural, or it can be created by building a dam, and a pond can be dug as easily as it can form on its own.
Fourth, how large is it, and how deep? Size alone rarely settles a name. A large pond can be bigger than a small lake, as explained below. Depth matters because it affects temperature, light, and life. A shallow pond warms through in summer, while a deep lake can stay cold at the bottom all year.
Fifth, is it connected or closed? Many bodies of water have both inflows and outflows: water arrives and water leaves. A closed basin has inflows but no outlet to the sea, so water leaves mainly by evaporation. That difference decides whether salts build up, whether water levels rise and fall sharply, and whether fish and other life can move in and out. Almost every confusion about bodies of water comes from applying only one of these questions, such as size, when the real difference lies in another, such as salt content or connection.
Saltwater Types: Oceans, Seas, Gulfs, Bays, and Straits
An ocean is the largest type of body of water: a vast, deep, salty water body that covers a large part of Earth’s surface. The five named oceans are the Pacific, Atlantic, Indian, Southern, and Arctic oceans, and together they form one connected world ocean, divided by continents and by convention into named parts. Oceans hold most of Earth’s water, and their depth, currents, and heat storage shape weather and climate far inland. Because this survey covers all types, it does not go into ocean depth, currents, or marine life in detail here.
A sea is a large salty water body, smaller than an ocean and usually partly enclosed by land. The Mediterranean Sea, the Caribbean Sea, and the Black Sea are familiar examples. Some seas are divisions of an ocean, marked off by islands, peninsulas, or currents rather than by a sharp physical boundary. Others are more enclosed, with only a narrow connection to the ocean. The word “sea” is also used loosely in everyday speech for the ocean in general, which is one reason the ocean and sea distinction confuses people. The working rule is size and enclosure: an ocean is the largest division, while a sea is a smaller, partly enclosed division of salty water.
A gulf is a large inlet of the ocean or a sea that is partly enclosed by land. The Gulf of Mexico is a well known example. A bay is a similar inlet, usually smaller and more open than a gulf, although the two words overlap and local usage varies. Chesapeake Bay and San Francisco Bay are familiar examples. Both gulfs and bays are parts of a larger salty water body, not separate water bodies in the same sense as a lake. Their sheltered shape matters: a partly enclosed inlet can warm up, calm down, and collect sediment differently from the open coast beside it, and that shelter often supports harbors, fisheries, and coastal towns.
Smaller inlets have their own names in different regions, including coves and fjords. A fjord is a long, narrow, deep inlet with steep sides, formed where a glacier carved a valley that later filled with seawater. Fjords in Norway are the classic example. They are still seawater inlets, but their depth and steep sides give them a character very different from a shallow, sandy bay.
A strait is a narrow body of water that connects two larger bodies of water. The Strait of Gibraltar connects the Atlantic Ocean to the Mediterranean Sea. A channel is a similar narrow waterway, and the word is also used for dredged or improved passages used by ships. The English Channel, between England and France, is a well known example. Straits and channels matter out of proportion to their size, because they control movement: ships, tides, currents, and migrating animals must pass through them, and a narrow passage can speed up tidal flow dramatically.
Still Freshwater: Lakes, Ponds, and Reservoirs
A lake is a large body of water surrounded by land. Most lakes are freshwater, but some are salty, especially in dry regions where evaporation leaves salts behind. Lakes form in many ways: glaciers can gouge out basins, volcanoes can leave craters that fill with water, rivers can leave cut off bends behind, and movements of Earth’s crust can create deep rift basins. The Great Lakes of North America, Lake Victoria in Africa, and Lake Baikal in Asia are among the largest and best known examples. Lake Baikal alone holds about 20 percent of all fresh surface water, according to the USGS, and about another 20 percent is stored in the Great Lakes. Those two examples show how concentrated fresh surface water is: a small number of very large lakes hold a large share of it.
A pond is a small, still body of water surrounded by land. In everyday speech, the difference between a pond and a lake is size: a pond is smaller and shallower than a lake. In practice there is no single official size or depth line that separates the two everywhere. Different regions, agencies, and traditions use different cutoffs, and some features called ponds are larger than some features called lakes. Depth is often the more useful difference. A pond is usually shallow enough that sunlight reaches most of the bottom and the water warms and mixes more evenly, while a deep lake can develop distinct layers of warm surface water and cold deep water.
A reservoir is a body of water stored behind a dam or in a constructed basin, usually created to supply drinking water, generate hydroelectric power, irrigate crops, or control floods. The USGS describes reservoirs as human made lakes, and that is the simplest definition. Some natural lakes are also used as reservoirs when a dam controls their outflow, which is one of the cases where a feature can be both natural and human made at once. Reservoirs differ from natural lakes mainly in how their water levels behave. A natural lake rises and falls mainly with weather and seasons. A reservoir rises and falls also with human decisions: water may be held back in a wet season and released in a dry season, or released suddenly to make room for floodwater or to generate power.
Flowing Freshwater: Rivers, Streams, and Their Networks
A river is a large, flowing body of water that moves across land in a channel, usually toward a lake, a sea, or an ocean. A stream is the same kind of feature at a smaller scale. In everyday speech, people use river for a large flow and stream, creek, brook, or beck for a smaller one, with the exact word varying by region. There is no single worldwide size line between a stream and a river. A named river in one region might carry less water than a named stream in another. The useful distinction is role, not just size: streams feed rivers, and rivers carry the combined flow onward.
Rivers begin in many places: a spring, a melting glacier, a lake outflow, or the joining of smaller streams in hills or mountains. The upper part of a river is usually steeper and faster, and the lower part is usually wider and slower, although waterfalls, rapids, and gorges break that pattern in places. Along the way, a river picks up water from tributaries, which are smaller streams or rivers that flow into it, and it may give off distributaries near its mouth, which are branches that split away from the main channel and carry water to a different part of a delta or floodplain.
The USGS notes that surface water includes streams of all sizes, from large rivers to small creeks. That range matters, because small streams do much of the collecting work. Rain and snowmelt first gather in tiny channels, which join into brooks, which join into streams, which join into rivers. By the time water reaches a great river, it has already passed through a whole network of smaller bodies of water. To understand a river, it helps to picture that network, not just the main channel shown in thick blue on a map.
Wetlands and In Between Waters
Wetlands are areas where water covers the soil, or is present at or near the surface, for at least part of the year. The USGS describes surface water as including freshwater wetlands, alongside lakes, reservoirs, ponds, and streams, and other federal sources describe wetlands more broadly as transitional areas between land and deepwater habitats. That transitional character is the point. A wetland is neither solid land nor open water in the usual sense. It is a place where the two meet, and where plants and animals adapted to wet conditions live.
Wetlands come in many forms, and their names vary by region. The main types are marshes, swamps, bogs, and fens, plus coastal wetlands such as salt marshes and mangrove swamps. Marshes are soft, wet areas, often dominated by grasses and reeds, with water that may be fresh, salty, or brackish. Swamps are wetlands dominated by trees or woody plants, standing in slow moving or still water. Bogs are wetlands fed mainly by rain, with acidic, nutrient poor water and thick peat formed from partly decayed plants. Fens are similar peat forming wetlands, but they are fed also by groundwater or surface water, so their water is usually less acidic and richer in minerals than a bog. Those differences in water source and chemistry explain much of the difference in plants and wildlife between wetland types.
An estuary is a partly enclosed coastal body of water where freshwater from rivers or streams mixes with saltwater from the ocean. NOAA describes estuaries in similar terms: bodies of water and their surrounding coastal habitats, typically found where rivers meet the sea, whose waters are brackish, a mixture of fresh water draining from the land and salty seawater. That brackish character is what defines an estuary. It is not fresh like a river, and not salty like the open sea. It changes with tides, river flow, and weather, so the same estuary can be fresher after heavy rain and saltier in a dry spell or at high tide.
Frozen Water: Glaciers, Ice Caps, and Icebergs
Glaciers and ice caps are sometimes left out of lists of bodies of water, because they are solid ice rather than liquid water. They belong in a complete survey anyway, because they are major stores of Earth’s freshwater, and because they feed liquid bodies of water when they melt. About two percent of Earth’s water is frozen in glaciers and ice caps, according to NOAA, and the USGS counts glaciers and ice caps as holding the largest share of Earth’s freshwater.
A glacier is a large, slow moving mass of ice formed from snow that has built up over many years, compressed into ice, and begun to flow under its own weight. An ice cap is a smaller, dome shaped cover of ice over land, while an ice sheet is the same idea at continental scale, as in Greenland and Antarctica. When glacier ice reaches the sea and breaks off, it forms icebergs, which are floating bodies of ice rather than bodies of water in the usual sense, but which matter to navigation and to ocean freshwater input as they melt.
Human Made Waters: Canals
People have made bodies of water for thousands of years, for transport, storage, defense, farming, and pleasure. Besides reservoirs, the main human made flowing type discussed here is the canal.
A canal is a human made channel built to carry water for transport, irrigation, or drainage. The USGS lists canals as human made streams, and that comparison is helpful: a canal behaves like a stream, but its course, depth, and flow are chosen by people rather than by landscape. Some canals connect two natural water bodies, such as two lakes or a river and a sea, and some carry water across land that would otherwise stay dry.
How Water Moves Between Bodies of Water
A watershed is the area of land that drains to a common outlet, such as the outflow of a reservoir, the mouth of a bay, or any point along a stream channel, as the USGS defines it. The word is sometimes used interchangeably with drainage basin or catchment. Ridges and hills that separate two watersheds are called the drainage divide. A watershed is not itself a body of water. It is the land area that collects water and delivers it to a body of water. That distinction matters, because people often confuse the two: the watershed is the land, and the river or lake is the water the land feeds.
Every body of water has a water balance: water in, water out, and change in storage. The USGS lists the main inflows to lakes and rivers as precipitation, overland runoff, groundwater seepage, and tributary inflows. The main outflows are evaporation, movement of water into groundwater, flow out through streams and rivers, and withdrawals by people. When inflows exceed outflows, water levels rise. When outflows exceed inflows, levels fall. When the two balance, levels stay roughly steady, even though water is constantly moving through.
A closed basin, also called an endorheic basin, is a watershed with no outlet to the sea. Water flows in, but it can only leave by evaporation or by seeping into the ground. The Great Salt Lake in Utah is a well known example in the United States: rivers flow into it, but no river flows out, so evaporation concentrates salts and the lake is far saltier than the rivers that feed it. Closed basins are common in dry regions, where rivers may never reach the sea because evaporation and seepage take the water first. They are a useful test of understanding: if a body of water has inflows but no outflow, expect its salt content and water level to respond strongly to climate, because evaporation is doing all the work of removal.
Why Names Can Mislead, and How to Choose the Right Word
For everyday use, a short decision guide helps. If the water is vast, deep, and salty, and it is one of Earth’s great divisions, call it an ocean. If it is large and salty but smaller and partly enclosed by land, call it a sea. If it is a partly enclosed inlet of salty water, call it a gulf or a bay, with gulf usually the larger and more enclosed of the two. If it is a narrow passage joining two larger waters, call it a strait or a channel. If the water is still, surrounded by land, and large, call it a lake. If it is still, surrounded by land, and small and shallow, call it a pond. If it is still water held behind a dam for human use, call it a reservoir, even if it looks like a lake. If the water flows in a channel, call it a river when it is large and a stream or creek when it is small, and remember that local usage may vary. If the land is wet for much of the year, with water at or near the surface and plants adapted to wet soil, call it a wetland, and then add the more specific term: marsh, swamp, bog, or fen. If fresh river water meets seawater in a partly enclosed coastal water, call it an estuary. If the water is frozen into a slow moving mass of ice on land, call it a glacier or an ice cap.
Frequently Asked Questions
What is the difference between an ocean and a sea?
An ocean is larger, deeper, and one of Earth’s great divisions of salty water. A sea is smaller and usually partly enclosed by land. The Mediterranean Sea and the Caribbean Sea are seas, while the Pacific and Atlantic are oceans. In everyday speech, people often say sea when they mean the ocean in general, but in geographical use the two words mark a difference in scale and enclosure.
What is the difference between a lake and a pond?
A pond is generally smaller and shallower than a lake, and sunlight often reaches most of a pond’s bottom. There is no single official size or depth line that separates the two in all places, so some ponds are larger than some lakes. Depth and local naming tradition often matter more than a strict size cutoff.
Is a river a body of water?
Yes. A body of water does not have to be still or contained. Rivers, streams, and canals are bodies of water because they are significant accumulations of water on Earth’s surface, even though the water in them is constantly moving and being replaced. The same applies to other flowing features, such as creeks and brooks.
What is the difference between a bay and a gulf?
Both are inlets of the ocean or a sea that are partly enclosed by land. A gulf is usually larger and more enclosed, while a bay is usually smaller and more open, but the two words overlap and local usage varies. The Gulf of Mexico and Chesapeake Bay show the usual pattern, rather than a strict rule.
What is brackish water?
Brackish water is water with a salt content between freshwater and seawater. It is typical of estuaries, where river water mixes with seawater, and of some lagoons and coastal wetlands. Its salt content changes with tides, river flow, and weather, so it is not a fixed value.
Is a wetland a body of water?
A wetland is a water related feature, but it is best described as a transitional area between land and open water, where soil is wet for much of the year. Some wetlands contain open ponds and channels, while others are mainly wet soil and plants with little open water. It is accurate to call wetlands bodies of water in a broad survey, and more precise to name the wetland type, such as marsh, swamp, bog, or fen, when the detail matters.
What is a watershed, and is it a body of water?
A watershed is not a body of water. It is the area of land that drains to a common outlet, such as a river, lake, or bay. Ridges and hills separate one watershed from another. The watershed feeds the body of water, but the land and the water are different things, and keeping them separate avoids a common confusion.
Which body of water holds the most freshwater that people can readily use?
Lakes and reservoirs hold the largest readily accessible stores of fresh surface water, with Lake Baikal and the Great Lakes together holding a very large share of fresh surface water. Rivers hold only a tiny share of total freshwater at any one moment, but they matter greatly because they keep delivering water from their watersheds. Groundwater and glaciers hold more freshwater overall, but they are harder to reach or are frozen, so lakes, rivers, and reservoirs remain the main day to day surface sources.
Sources & references
- U.S. Geological Survey, Water Science School, "Freshwater (Lakes and Rivers) and the Water Cycle". https://www.usgs.gov/water-science-school/science/freshwater-lakes-and-rivers-and-water-cycle
- National Oceanic and Atmospheric Administration, National Ocean Service, "How much water is in the ocean?" https://oceanservice.noaa.gov/facts/oceanwater.html
- National Oceanic and Atmospheric Administration, National Ocean Service, "Where is all of the Earth's water?" https://oceanservice.noaa.gov/facts/wherewater.html
- U.S. Geological Survey, Water Science School, "Watersheds and Drainage Basins". https://www.usgs.gov/water-science-school/science/watersheds-and-drainage-basins
- National Oceanic and Atmospheric Administration, National Ocean Service, "Estuaries Tutorial". https://oceanservice.noaa.gov/education/tutorial_estuaries/
