What is streamflow? How do streams get their water? To learn about streamflow and its role in the water cycle, continue reading.
Note: This section of the Water Science School discusses the Earth's "natural" water cycle without human interference.
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Water cycle components » Atmosphere · Condensation · Evaporation · Evapotranspiration · Freshwater lakes and rivers · Groundwater flow · Groundwater storage · Ice and snow · Infiltration · Oceans · Precipitation · Snowmelt · Springs · Streamflow · Sublimation · Surface runoff
Streamflow and the Water Cycle

Credit: Wikipedia
If you read our discussion on the role the oceans play in the water cycle, you know that evaporation from the oceans is the primary way that water returns to the atmosphere from the Earth's surface. Water returns to the Earth from precipitation falling on the land, where gravity either takes it into the ground as infiltration or it begins running downhill as surface runoff. But how does much of the water get back into the oceans to keep the water cycle going? A lot of runoff ends up in creeks, streams, and rivers, flowing downhill towards the oceans. Unless the river flows into a closed lake, a rare occurrence, or is diverted for humans' uses, a common occurrence, they empty into the oceans, thus fulfilling their water-cycle duties.
The U.S. Geological Survey (USGS) uses the term "streamflow" to refer to the amount of water flowing in a river. Although USGS usually uses the term "stream" when discussing flowing water bodies, in these pages we'll use "rivers" more often, since that is probably what you are more familiar with.
Importance of rivers
Rivers are invaluable to not only people, but to life everywhere. Not only are rivers a great place for people (and their dogs) to play, but people use river water for drinking-water supplies and irrigation water, to produce electricity, to flush away wastes (hopefully, but not always, treated wastes), to transport merchandise, and to obtain food. Rivers are major aquatic landscapes for all manners of plants and animals. Rivers even help keep the aquifers underground full of water by discharging water downward through their streambeds. And, we've already mentioned that the oceans stay full of water because rivers and runoff continually refreshes them.
Watersheds and rivers
One word can explain why any river exists on Earth—gravity. You've heard that "water seeks its own level," but really water is seeking the center of the Earth, just like everything else. In practical terms, water generally seeks to flow to the oceans, which are at sea level. So, no matter where on Earth water is, it tries to flow downhill. Because the Earth is not a very level place, water ends up occupying the valleys and depressions in the landscape as rivers and lakes.
When looking at the location of rivers and the amount of streamflow in rivers, the key concept is the river's "watershed". What is a watershed? Easy, if you are standing on the ground right now, just look down. You're standing, and everyone is standing, in a watershed. A watershed is the area of land where all of the water that falls in it and drains off of it goes to the same place. Watersheds can be as small as a footprint or large enough to encompass all the land that drains water into rivers that drain into Chesapeake Bay, where it enters the Atlantic Ocean. Larger watersheds contain many smaller watersheds. It all depends on the outflow point; all of the land that drains water to the outflow point is the watershed for that outflow location. Watersheds are important because the streamflow and the water quality of a river are affected by things, human-induced or not, happening in the land area "above" the river-outflow point
Streamflow is always changing
Streamflow is always changing, from day to day and even minute to minute. Of course, the main influence on streamflow is precipitation runoff in the watershed. Rainfall causes rivers to rise, and a river can even rise if it only rains very far up in the watershed - remember that water that falls in a watershed will eventually drain by the outflow point.

Credit: Howard Perlman, USGS
The size of a river is highly dependent on the size of its watershed. Large rivers have watersheds with lots of surface area; small rivers have smaller watersheds. Likewise, different size rivers react differently to storms and rainfall. Large rivers rise and fall slower and at a slower rate than small rivers. In a small watershed, a storm can cause 100 times as much water to flow by each minute as during base-periods, but the river will rise and fall possibly in a matter of minutes and hours. Large rivers may take days to rise and fall, and flooding can last for a number of days. After all, it can take days for all the water that fell hundreds of miles upstream to drain past an outflow point.
If you have ever wondered how many gallons of water falls during a storm, use our interactive rainfall calculator to find out.
Hydrologists study streamflows with hydrographs
USGS uses a hydrograph to study streamflow in rivers. A hydrograph is a chart showing, most often, river stage (height of the water above an arbitrary altitude) and streamflow (amount of water, usually in cubic feet per second). Other properties, such as rainfall and water-quality parameters can also be plotted. The hydrograph below shows rainfall and streamflow for a single day for Peachtree Creek at Atlanta, Georgia (USGS station number 02336300).
Precipitation influences streamflow
On Dec. 24, 2002, about two inches of rainfall fell in the Peachtree Creek watershed. This provides a good example to describe streamflow characteristics during a storm since the rain fell for only a few hours on that day and Peachtree Creek was at base-flow conditions before the rain started. The chart below shows rainfall, in inches, during each 15-minute increment on Dec. 24th and the continuous measure of streamflow, in cubic feet per second (ft3/s).
The brown line in the chart shows that streamflow is much higher during the flood period than just before it. The line shows that the baseflow was about 50 ft3/s before the river started to rise, but that just a few hours later, at 9:00 AM streamflow was over 6, 000 ft3/s - that is about 150 times the amount of water flowing by as during baseflow conditions. This is characteristic of small streams, especially urban streams where runoff enters the river very quickly.
It is possible to estimate the total amount of water that flowed during Dec. 24, 2002, and compare it to a day when the streamflows are at base-flow conditions (stream stage of about 2.81 feet). At base flow, an estimated 27,800,000 gallons of water will flow by the Peachtree Creek measurement station in one day. Using mean streamflows for each 15-minute period during the storm of Dec. 24th, an estimated 4,290,000,000 gallons flowed by. That would be about 154 times more water than during a day of base flow.
Pre rainfall:
Stream stage: 2.81 feet
Streamflow: 43 cubic feet per second
Post rainfall:
Stream stage: 17.33 feet
Streamflow: 6,630 cubic feet per second
Mechanisms that cause changes in streamflow
Rivers are always moving, which is good for everything, as stagnant water doesn't stay fresh and inviting very long. There are many factors, both natural and human-induced, that cause rivers to continuously change:
Natural mechanisms
- Runoff from rainfall and snowmelt
- Evaporation from soil and surface-water bodies
- Transpiration by vegetation
- Ground-water discharge from aquifers
- Ground-water recharge from surface-water bodies
- Sedimentation of lakes and wetlands
- Formation or dissipation of glaciers, snowfields, and permafrost
Human-induced mechanisms
- Surface-water withdrawals and transbasin diversions
- River-flow regulation for hydropower and navigation
- Construction, removal, and sedimentation of reservoirs and stormwater detention ponds
- Stream channelization and levee construction
- Drainage or restoration of wetlands
- Land-use changes such as urbanization that alter rates of erosion, infiltration, overland flow, or evapotranspiration
- Wastewater outfalls
- Irrigation wastewater return flow
Streamflow and global water distribution
Even though the water flowing in rivers is tremendously valuable to not only people but also to much of life on Earth, it makes up just a miniscule amount of Earth's water. Considering just the freshwater on Earth, streamflow in rivers only accounts for about six-one thousands of one percent (0.006%)! The first table below shows that about 0.002 percent of all Earth's water is contained in rivers, and only 0.006 percent of the world's freshwater is in rivers.
Water source | Water volume, in cubic miles |
Water volume, in cubic kilometers |
Percent of total water | Percent of total freshwater |
---|---|---|---|---|
Fresh groundwater | 2,526,000 | 10,530,000 | 0.8% | 30.1% |
Groundwater | 5,614,000 | 23,400,000 | 1.7% | -- |
Total global water | 332,500,000 | 1,386,000,000 | -- | -- |
Source: Gleick, P. H., 1996: Water resources. In Encyclopedia of Climate and Weather, ed. by S. H. Schneider, Oxford University Press, New York, vol. 2, pp. 817-823.
Want to know more about streamflow and the water cycle? Follow me to the Streamgaging Basics website!
More topics and other components of the water cycle:
The Fundamentals of the Water Cycle
The Water Cycle for Adults and Advanced Students
Precipitation and the Water Cycle
A Comprehensive Study of the Natural Water Cycle
Streamflow and the Water Cycle
Snowmelt Runoff and the Water Cycle
Evaporation and the Water Cycle
Springs and the Water Cycle
Sublimation and the Water Cycle
The Atmosphere and the Water Cycle
Surface Runoff and the Water Cycle
Infiltration and the Water Cycle
Condensation and the Water Cycle
- Overview
What is streamflow? How do streams get their water? To learn about streamflow and its role in the water cycle, continue reading.
Note: This section of the Water Science School discusses the Earth's "natural" water cycle without human interference.
• Water Science School HOME • Surface Water topics • The Water Cycle •
Water cycle components » Atmosphere · Condensation · Evaporation · Evapotranspiration · Freshwater lakes and rivers · Groundwater flow · Groundwater storage · Ice and snow · Infiltration · Oceans · Precipitation · Snowmelt · Springs · Streamflow · Sublimation · Surface runoff
Streamflow and the Water Cycle
Sources/Usage: Some content may have restrictions. Visit Media to see details.West Branch Susquehanna River, Pennsylvania
Credit: WikipediaIf you read our discussion on the role the oceans play in the water cycle, you know that evaporation from the oceans is the primary way that water returns to the atmosphere from the Earth's surface. Water returns to the Earth from precipitation falling on the land, where gravity either takes it into the ground as infiltration or it begins running downhill as surface runoff. But how does much of the water get back into the oceans to keep the water cycle going? A lot of runoff ends up in creeks, streams, and rivers, flowing downhill towards the oceans. Unless the river flows into a closed lake, a rare occurrence, or is diverted for humans' uses, a common occurrence, they empty into the oceans, thus fulfilling their water-cycle duties.
The U.S. Geological Survey (USGS) uses the term "streamflow" to refer to the amount of water flowing in a river. Although USGS usually uses the term "stream" when discussing flowing water bodies, in these pages we'll use "rivers" more often, since that is probably what you are more familiar with.
Importance of rivers
Rivers are invaluable to not only people, but to life everywhere. Not only are rivers a great place for people (and their dogs) to play, but people use river water for drinking-water supplies and irrigation water, to produce electricity, to flush away wastes (hopefully, but not always, treated wastes), to transport merchandise, and to obtain food. Rivers are major aquatic landscapes for all manners of plants and animals. Rivers even help keep the aquifers underground full of water by discharging water downward through their streambeds. And, we've already mentioned that the oceans stay full of water because rivers and runoff continually refreshes them.
Watersheds and rivers
One word can explain why any river exists on Earth—gravity. You've heard that "water seeks its own level," but really water is seeking the center of the Earth, just like everything else. In practical terms, water generally seeks to flow to the oceans, which are at sea level. So, no matter where on Earth water is, it tries to flow downhill. Because the Earth is not a very level place, water ends up occupying the valleys and depressions in the landscape as rivers and lakes.
When looking at the location of rivers and the amount of streamflow in rivers, the key concept is the river's "watershed". What is a watershed? Easy, if you are standing on the ground right now, just look down. You're standing, and everyone is standing, in a watershed. A watershed is the area of land where all of the water that falls in it and drains off of it goes to the same place. Watersheds can be as small as a footprint or large enough to encompass all the land that drains water into rivers that drain into Chesapeake Bay, where it enters the Atlantic Ocean. Larger watersheds contain many smaller watersheds. It all depends on the outflow point; all of the land that drains water to the outflow point is the watershed for that outflow location. Watersheds are important because the streamflow and the water quality of a river are affected by things, human-induced or not, happening in the land area "above" the river-outflow point
Streamflow is always changing
Streamflow is always changing, from day to day and even minute to minute. Of course, the main influence on streamflow is precipitation runoff in the watershed. Rainfall causes rivers to rise, and a river can even rise if it only rains very far up in the watershed - remember that water that falls in a watershed will eventually drain by the outflow point.
Sources/Usage: Public Domain. Visit Media to see details.This is a typical flood on Peachtree Creek, shown in "before and after" pictures from the homeowner's 10-foot high entryway. The flood picture (on the right) was taken on May 6, 2003 in the late afternoon when stream stage was about 17 feet. The flood peaked that day at 7:30 PM Eastern Time in the evening, when the stream stage reached 17.77 feet with a corresponding instantaneous streamflow of 6,960 cubic feet per second (cfs). Alternatively, base flow at Peachtree Creek (left picture) is around 2.5 feet, with a streamflow of about 25 cfs.
Credit: Howard Perlman, USGSThe size of a river is highly dependent on the size of its watershed. Large rivers have watersheds with lots of surface area; small rivers have smaller watersheds. Likewise, different size rivers react differently to storms and rainfall. Large rivers rise and fall slower and at a slower rate than small rivers. In a small watershed, a storm can cause 100 times as much water to flow by each minute as during base-periods, but the river will rise and fall possibly in a matter of minutes and hours. Large rivers may take days to rise and fall, and flooding can last for a number of days. After all, it can take days for all the water that fell hundreds of miles upstream to drain past an outflow point.
If you have ever wondered how many gallons of water falls during a storm, use our interactive rainfall calculator to find out.
Hydrologists study streamflows with hydrographs
USGS uses a hydrograph to study streamflow in rivers. A hydrograph is a chart showing, most often, river stage (height of the water above an arbitrary altitude) and streamflow (amount of water, usually in cubic feet per second). Other properties, such as rainfall and water-quality parameters can also be plotted. The hydrograph below shows rainfall and streamflow for a single day for Peachtree Creek at Atlanta, Georgia (USGS station number 02336300).
Precipitation influences streamflow
On Dec. 24, 2002, about two inches of rainfall fell in the Peachtree Creek watershed. This provides a good example to describe streamflow characteristics during a storm since the rain fell for only a few hours on that day and Peachtree Creek was at base-flow conditions before the rain started. The chart below shows rainfall, in inches, during each 15-minute increment on Dec. 24th and the continuous measure of streamflow, in cubic feet per second (ft3/s).
The brown line in the chart shows that streamflow is much higher during the flood period than just before it. The line shows that the baseflow was about 50 ft3/s before the river started to rise, but that just a few hours later, at 9:00 AM streamflow was over 6, 000 ft3/s - that is about 150 times the amount of water flowing by as during baseflow conditions. This is characteristic of small streams, especially urban streams where runoff enters the river very quickly.
It is possible to estimate the total amount of water that flowed during Dec. 24, 2002, and compare it to a day when the streamflows are at base-flow conditions (stream stage of about 2.81 feet). At base flow, an estimated 27,800,000 gallons of water will flow by the Peachtree Creek measurement station in one day. Using mean streamflows for each 15-minute period during the storm of Dec. 24th, an estimated 4,290,000,000 gallons flowed by. That would be about 154 times more water than during a day of base flow.
Pre rainfall:
Stream stage: 2.81 feet
Streamflow: 43 cubic feet per secondPost rainfall:
Stream stage: 17.33 feet
Streamflow: 6,630 cubic feet per secondMechanisms that cause changes in streamflow
Rivers are always moving, which is good for everything, as stagnant water doesn't stay fresh and inviting very long. There are many factors, both natural and human-induced, that cause rivers to continuously change:
Natural mechanisms
- Runoff from rainfall and snowmelt
- Evaporation from soil and surface-water bodies
- Transpiration by vegetation
- Ground-water discharge from aquifers
- Ground-water recharge from surface-water bodies
- Sedimentation of lakes and wetlands
- Formation or dissipation of glaciers, snowfields, and permafrost
Human-induced mechanisms
- Surface-water withdrawals and transbasin diversions
- River-flow regulation for hydropower and navigation
- Construction, removal, and sedimentation of reservoirs and stormwater detention ponds
- Stream channelization and levee construction
- Drainage or restoration of wetlands
- Land-use changes such as urbanization that alter rates of erosion, infiltration, overland flow, or evapotranspiration
- Wastewater outfalls
- Irrigation wastewater return flow
Streamflow and global water distribution
Even though the water flowing in rivers is tremendously valuable to not only people but also to much of life on Earth, it makes up just a miniscule amount of Earth's water. Considering just the freshwater on Earth, streamflow in rivers only accounts for about six-one thousands of one percent (0.006%)! The first table below shows that about 0.002 percent of all Earth's water is contained in rivers, and only 0.006 percent of the world's freshwater is in rivers.
One estimate of global water distribution Water source Water volume,
in cubic milesWater volume,
in cubic kilometersPercent of total water Percent of total freshwater Fresh groundwater 2,526,000 10,530,000 0.8% 30.1% Groundwater 5,614,000 23,400,000 1.7% -- Total global water 332,500,000 1,386,000,000 -- -- Source: Gleick, P. H., 1996: Water resources. In Encyclopedia of Climate and Weather, ed. by S. H. Schneider, Oxford University Press, New York, vol. 2, pp. 817-823.
Want to know more about streamflow and the water cycle? Follow me to the Streamgaging Basics website!
- Science
More topics and other components of the water cycle:
The Fundamentals of the Water Cycle
Earth's water is always in movement, and the natural water cycle, also known as the hydrologic cycle, describes the continuous movement of water on, above, and below the surface of the Earth. Water is always changing states between liquid, vapor, and ice, with these processes happening in the blink of an eye and over millions of years.Filter Total Items: 18The Water Cycle for Adults and Advanced Students
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A spring is a place where water moving underground finds an opening to the land surface and emerges, sometimes as just a trickle, maybe only after a rain, and sometimes in a continuous flow. Spring water can also emerge from heated rock underground, giving rise to hot springs, which people have found to make a delightful way of soaking away their problems. Note: This section of the Water Science...Sublimation and the Water Cycle
Solid, liquid, and gas - the three states of water. We see water freeze and turn to ice and we see water evaporate and turn to gas but... have you ever seen ice evaporate directly to gas? This process is called sublimation and you can read all about it below. Note: This section of the Water Science School discusses the Earth's "natural" water cycle without human interference.The Atmosphere and the Water Cycle
The atmosphere is the superhighway in the sky that moves water everywhere over the Earth. Water at the Earth's surface evaporates into water vapor which rises up into the sky to become part of a cloud which will float off with the winds, eventually releasing water back to Earth as precipitation. Note: This section of the Water Science School discusses the Earth's "natural" water cycle without...Surface Runoff and the Water Cycle
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The air is full of water, as water vapor, even if you can't see it. Condensation is the process of water vapor turning back into liquid water, with the best example being those big, fluffy clouds floating over your head. And when the water droplets in clouds combine, they become heavy enough to form raindrops to rain down onto your head. Note: This section of the Water Science School discusses the...