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Streamflow, or channel runoff, is the flow of
water Water (chemical formula ) is an inorganic, transparent, tasteless, odorless, and nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the fluids of all known living organisms (in which it acts as a ...
in
stream A stream is a continuous body of water, body of surface water Current (stream), flowing within the stream bed, bed and bank (geography), banks of a channel (geography), channel. Depending on its location or certain characteristics, a stream ...
s and other
channel Channel, channels, channeling, etc., may refer to: Geography * Channel (geography), in physical geography, a landform consisting of the outline (banks) of the path of a narrow body of water. Australia * Channel Country, region of outback Austral ...
s, and is a major element of the
water cycle The water cycle, also known as the hydrologic cycle or the hydrological cycle, is a biogeochemical cycle that describes the continuous movement of water on, above and below the surface of the Earth. The mass of water on Earth remains fairly cons ...
. It is one component of the movement of water from the land to
waterbodies A body of water or waterbody (often spelled water body) is any significant accumulation of water on the surface of Earth or another planet. The term most often refers to oceans, seas, and lakes, but it includes smaller pools of water such as p ...
, the other component being
surface runoff Surface runoff (also known as overland flow) is the flow of water occurring on the ground surface when excess rainwater, stormwater, meltwater, or other sources, can no longer sufficiently rapidly infiltrate in the soil. This can occur when th ...
. Water flowing in channels comes from surface runoff from adjacent hillslopes, from
groundwater Groundwater is the water present beneath Earth's surface in rock and soil pore spaces and in the fractures of rock formations. About 30 percent of all readily available freshwater in the world is groundwater. A unit of rock or an unconsolidate ...
flow out of the ground, and from water discharged from pipes. The discharge of water flowing in a channel is measured using
stream gauge A stream gauge, streamgage or stream gauging station is a location used by hydrologists or environmental scientists to monitor and test terrestrial bodies of water. Hydrometric measurements of water level surface elevation ("stage") and/or volu ...
s or can be estimated by the
Manning equation The Manning formula or Manning's equation is an Empirical relationship, empirical formula estimating the average velocity of a liquid flowing in a conduit that does not completely enclose the liquid, i.e., open channel flow. However, this equation i ...
. The record of flow over time is called a
hydrograph A hydrograph is a graph showing the rate of flow ( discharge) versus time past a specific point in a river, channel, or conduit carrying flow. The rate of flow is typically expressed in cubic meters or cubic feet per second (cms or cfs). It can al ...
.
Flood A flood is an overflow of water ( or rarely other fluids) that submerges land that is usually dry. In the sense of "flowing water", the word may also be applied to the inflow of the tide. Floods are an area of study of the discipline hydrol ...
ing occurs when the volume of water exceeds the capacity of the channel.


Role in the water cycle

Streams play a critical role in the
hydrologic cycle The water cycle, also known as the hydrologic cycle or the hydrological cycle, is a biogeochemical cycle that describes the continuous movement of water on, above and below the surface of the Earth. The mass of water on Earth remains fairly const ...
that is essential for all life on Earth. A diversity of biological species, from unicellular organisms to vertebrates, depend on flowing-water systems for their
habitat In ecology, the term habitat summarises the array of resources, physical and biotic factors that are present in an area, such as to support the survival and reproduction of a particular species. A species habitat can be seen as the physical ...
and food resources.
Rivers A river is a natural flowing watercourse, usually freshwater, flowing towards an ocean, sea, lake or another river. In some cases, a river flows into the ground and becomes dry at the end of its course without reaching another body of wate ...
are major aquatic landscapes for all manners of plants and animals. Rivers even help keep the
aquifers An aquifer is an underground layer of water-bearing, permeable rock, rock fractures, or unconsolidated materials (gravel, sand, or silt). Groundwater from aquifers can be extracted using a water well. Aquifers vary greatly in their characterist ...
underground full of water by discharging water downward through their streambeds. In addition to that, the oceans stay full of water because rivers and runoff continually refreshes them. Streamflow is the main mechanism by which water moves from the land to the oceans or to basins of interior drainage.


Sources

Stream discharge is derived from four sources: channel precipitation, overland flow, interflow, and groundwater. *Channel
precipitation In meteorology, precipitation is any product of the condensation of atmospheric water vapor that falls under gravitational pull from clouds. The main forms of precipitation include drizzle, rain, sleet, snow, ice pellets, graupel and hail. ...
is the moisture falling directly on the water surface, and in most streams, it adds very little to discharge. *
Groundwater Groundwater is the water present beneath Earth's surface in rock and soil pore spaces and in the fractures of rock formations. About 30 percent of all readily available freshwater in the world is groundwater. A unit of rock or an unconsolidate ...
enters the streambed where the channel intersects the water table, providing a steady supply of water, termed baseflow, during both dry and rainy periods. Because of the large supply of groundwater available to the streams and the slowness of the response of groundwater to precipitation events, baseflow changes only gradually over time, and it is rarely the main cause of flooding. However, it does contribute to flooding by providing a stage onto which runoff from other sources is superimposed. *
Interflow In hydrology, interflow is the lateral movement of water in the unsaturated zone, or vadose zone, that first returns to the surface or enters a stream prior to becoming groundwater. Interflow is sometimes used interchangeably with throughflow; howev ...
is water that infiltrates the soil and then moves laterally to the stream channel in the zone above the
water table The water table is the upper surface of the zone of saturation. The zone of saturation is where the pores and fractures of the ground are saturated with water. It can also be simply explained as the depth below which the ground is saturated. T ...
. Much of this water is transmitted within the soil, some of it moving within the horizons. Next to
baseflow Baseflow (also called drought flow, groundwater recession flow, low flow, low-water flow, low-water discharge and sustained or fair-weather runoff) is the portion of the streamflow that is sustained between precipitation events, fed to streams by d ...
, it is the most important source of discharge for streams in forested lands. Overland flow in heavily forested areas makes negligible contributions to streamflow. *In dry regions, cultivated, and urbanized areas, overland flow or
surface runoff Surface runoff (also known as overland flow) is the flow of water occurring on the ground surface when excess rainwater, stormwater, meltwater, or other sources, can no longer sufficiently rapidly infiltrate in the soil. This can occur when th ...
is usually a major source of streamflow. Overland flow is a
stormwater Stormwater, also spelled storm water, is water that originates from precipitation (storm), including heavy rain and meltwater from hail and snow. Stormwater can soak into the soil ( infiltrate) and become groundwater, be stored on depressed la ...
runoff that begins as thin layer of water that moves very slowly (typically less than 0.25 feet per second) over the ground. Under intensive rainfall and in the absence of barriers such as rough ground, vegetation, and absorbing soil, it can mount up, rapidly reaching stream channels in minutes and causing sudden rises in discharge. The quickest response times between rainfall and streamflow occur in urbanized areas where yard drains, street gutters, and
storm sewers A storm drain, storm sewer (United Kingdom, U.S. and Canada), surface water drain/sewer (United Kingdom), or stormwater drain ( Australia and New Zealand) is infrastructure designed to drain excess rain and ground water from impervious surface ...
collect overland flow and route it to streams straightaway. Runoff velocities in storm sewer pipes can reach 10 to 15 feet per second.


Mechanisms that cause changes in streamflow

Rivers are always moving, which is good for environment, as stagnant water does not 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 Evaporation is a type of vaporization that occurs on the surface of a liquid as it changes into the gas phase. High concentration of the evaporating substance in the surrounding gas significantly slows down evaporation, such as when humidi ...
from soil and surface-water bodies *
Transpiration Transpiration is the process of water movement through a plant and its evaporation from aerial parts, such as leaves, stems and flowers. Water is necessary for plants but only a small amount of water taken up by the roots is used for growth a ...
by vegetation * Ground-water discharge from aquifers * Ground-water recharge from surface-water bodies *
Sedimentation Sedimentation is the deposition of sediments. It takes place when particles in suspension settle out of the fluid in which they are entrained and come to rest against a barrier. This is due to their motion through the fluid in response to the ...
of
lakes A lake is an area filled with water, localized in a basin, surrounded by land, and distinct from any river or other outlet that serves to feed or drain the lake. Lakes lie on land and are not part of the ocean, although, like the much larger ...
and
wetlands A wetland is a distinct ecosystem that is flooded or saturated by water, either permanently (for years or decades) or seasonally (for weeks or months). Flooding results in oxygen-free (anoxic) processes prevailing, especially in the soils. The ...
* Formation or dissipation of
glaciers A glacier (; ) is a persistent body of dense ice that is constantly moving under its own weight. A glacier forms where the accumulation of snow exceeds its ablation over many years, often centuries. It acquires distinguishing features, such as ...
, snowfields, and
permafrost Permafrost is ground that continuously remains below 0 °C (32 °F) for two or more years, located on land or under the ocean. Most common in the Northern Hemisphere, around 15% of the Northern Hemisphere or 11% of the global surface ...
Human-induced mechanisms * Surface-water withdrawals and transbasin diversions * River-flow regulation for hydropower and navigation * Construction, removal, and sedimentation of reservoirs and stormwater retention ponds * Stream channelization and
levee A levee (), dike (American English), dyke (English in the Commonwealth of Nations, Commonwealth English), embankment, floodbank, or stop bank is a structure that is usually soil, earthen and that often runs parallel (geometry), parallel to ...
construction * Drainage or restoration of wetlands *
Land use Land use involves the management and modification of natural environment or wilderness into built environment such as settlements and semi-natural habitats such as arable fields, pastures, and managed woods. Land use by humans has a long h ...
changes such as
urbanization Urbanization (or urbanisation) refers to the population shift from rural to urban areas, the corresponding decrease in the proportion of people living in rural areas, and the ways in which societies adapt to this change. It is predominantly t ...
that alter rates of erosion, infiltration, overland flow, or
evapotranspiration Evapotranspiration (ET) is the combined processes by which water moves from the earth’s surface into the atmosphere. It covers both water evaporation (movement of water to the air directly from soil, canopies, and water bodies) and transpi ...
*
Wastewater Wastewater is water generated after the use of freshwater, raw water, drinking water or saline water in a variety of deliberate applications or processes. Another definition of wastewater is "Used water from any combination of domestic, industr ...
outfalls *
Irrigation Irrigation (also referred to as watering) is the practice of applying controlled amounts of water to land to help grow Crop, crops, Landscape plant, landscape plants, and Lawn, lawns. Irrigation has been a key aspect of agriculture for over 5,00 ...


Measurement

Streamflow is measured as an amount of water passing through a specific point over time. The units used in the United States are ''cubic feet per second'', while in most other countries ''cubic meters per second'' are utilized. There are a variety of ways to measure the discharge of a stream or canal. A stream gauge provides continuous flow over time at one location for water resource and environmental management or other purposes. Streamflow values are better indicators than gage height of conditions along the whole river. Measurements of streamflow are made about every six weeks by
United States Geological Survey The United States Geological Survey (USGS), formerly simply known as the Geological Survey, is a scientific agency of the United States government. The scientists of the USGS study the landscape of the United States, its natural resources, ...
(USGS) personnel. They wade into the stream to make the measurement or do so from a boat, bridge, or cableway over the stream. For each gaging station, a relation between gage height and streamflow is determined by simultaneous measurements of gage height and streamflow over the natural range of flows (from very low flows to floods). This relation provides the streamflow data from that station. For purposes that do not require a continuous measurement of stream flow over time,
current Currents, Current or The Current may refer to: Science and technology * Current (fluid), the flow of a liquid or a gas ** Air current, a flow of air ** Ocean current, a current in the ocean *** Rip current, a kind of water current ** Current (stre ...
meters or acoustic Doppler velocity profilers can be used. For small streams — a few meters wide or smaller —
weir A weir or low head dam is a barrier across the width of a river that alters the flow characteristics of water and usually results in a change in the height of the river level. Weirs are also used to control the flow of water for outlets of l ...
s may be installed.


Approximation

One informal method that provides an approximation of the stream flow termed the Orange Method or Float Method is: # Measure a length of stream, and mark the start and finish points. The longest length without changing stream conditions is desired to obtain the most accurate measurement. # Place an orange at the starting point and measure the time for it to reach the finish point with a stopwatch. Repeat this at least three times and average the measurement times. # Express velocity in meters per second. If the measurements were made at midstream (maximum velocity), the mean stream velocity is approximately 0.8 of the measured velocity for rough (rocky) bottom conditions and 0.9 of the measured velocity for smooth (mud, sand, smooth bedrock) bottom conditions.


Monitoring

In the United States, streamflow gauges are funded primarily from state and local government funds. In Fiscal Year 2008 the USGS provided 35% of the funding for everyday operation and maintenance of gauges. Additionally, USGS uses hydrographs to study streamflow in rivers. A
hydrograph A hydrograph is a graph showing the rate of flow ( discharge) versus time past a specific point in a river, channel, or conduit carrying flow. The rate of flow is typically expressed in cubic meters or cubic feet per second (cms or cfs). It can al ...
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 Water quality refers to the chemical, physical, and biological characteristics of water based on the standards of its usage. It is most frequently used by reference to a set of standards against which compliance, generally achieved through tr ...
parameters can also be plotted.


Forecasting

For most streams especially those with a small watershed, no record of discharge is available. In that case, it is possible to make discharge estimates using the rational method or some modified version of it. However, if chronological records of discharge are available for a stream, a short term forecast of discharge can be made for a given rainstorm using a hydrograph. Unit hydrograph method. This method involves building a graph in which the discharge generated by a rainstorm of a given size is plotted over time, usually hours or days. It is called the unit hydrograph method because it addresses only the runoff produced by a particular rainstorm in a specified period of time—the time taken for a river to rise, peak, and fall in response to a storm. Once a rainfall-runoff relationship is established, then subsequent rainfall data can be used to forecast streamflow for selected storms, called standard storms. A standard rainstorm is a high intensity storm of some known magnitude and frequency. One method of unit hydrograph analysis involves expressing the hour by hour or day by day increase in streamflow as a percentage of total runoff. Plotted on a graph, these data from the unit hydrograph for that storm, which represents the runoff added to the pre-storm baseflow. To forecast the flows in a large drainage basin using the unit hydrograph method would be difficult because in a large basin geographic conditions may vary significantly from one part of the basin to another. This is especially so with the distribution of rainfall because an individual rainstorm rarely covers the basin evenly. As a result, the basin does not respond as a unit to a given storm, making it difficult to construct a reliable hydrograph. Magnitude and frequency method. For large basins, where unit hydrograph might not be useful and reliable, the magnitude and frequency method is used to calculate the probability of recurrence of large flows based on records of past years’ flows. In United States, these records are maintained by the Hydrological Division of the USGS for large streams. For a basin with an area of 5,000 square miles or more, the river system is typically gauged at five to ten places. The data from each gauging station apply to the part of the basin upstream that location. Given several decades of peak annual discharges for a river, limited projections can be made to estimate the size of some large flow that has not been experienced during the period of record. The technique involves projecting the curve (graph line) formed when peak annual discharges are plotted against their respective recurrence intervals. However, in most cases the curve bends strongly, making it difficult to plot a projection accurately. This problem can be overcome by plotting the discharge and/or recurrence interval data on logarithmic graph paper. Once the plot is straightened, a line can be ruled drawn through the points. A projection can then be made by extending the line beyond the points and then reading the appropriate discharge for the recurrence interval in question.


Relationship to the environment

Runoff of water in channels is responsible for transport of
sediment Sediment is a naturally occurring material that is broken down by processes of weathering and erosion, and is subsequently transported by the action of wind, water, or ice or by the force of gravity acting on the particles. For example, sand an ...
,
nutrient A nutrient is a substance used by an organism to survive, grow, and reproduce. The requirement for dietary nutrient intake applies to animals, plants, fungi, and protists. Nutrients can be incorporated into cells for metabolic purposes or excret ...
s, and
pollution Pollution is the introduction of contaminants into the natural environment that cause adverse change. Pollution can take the form of any substance (solid, liquid, or gas) or energy (such as radioactivity, heat, sound, or light). Pollutants, the ...
downstream. Without streamflow, the water in a given watershed would not be able to naturally progress to its final destination in a lake or ocean. This would disrupt the ecosystem. Streamflow is one important route of water from the land to lakes and oceans. The other main routes are surface runoff (the flow of water from the land into nearby watercourses that occurs during precipitation and as a result of irrigation), flow of groundwater into surface waters, and the flow of water from constructed pipes and channels.


Relationship to society

Streamflow confers on society both benefits and hazards. Runoff downstream is a means to collect water for storage in
dams A dam is a barrier that stops or restricts the flow of surface water or underground streams. Reservoirs created by dams not only suppress floods but also provide water for activities such as irrigation, human consumption, industrial use, ...
for
power generation Electricity generation is the process of generating electric power from sources of primary energy. For utilities in the electric power industry, it is the stage prior to its delivery ( transmission, distribution, etc.) to end users or its stor ...
of water abstraction. The flow of water assists transport downstream. A given watercourse has a maximum streamflow rate that can be accommodated by the channel that can be calculated. If the streamflow exceeds this maximum rate, as happens when an excessive amount of water is present in the watercourse, the channel cannot handle all the water, and flooding occurs. The 1993 Mississippi river flood, the largest ever recorded on the river, was a response to a heavy, long duration spring and summer rainfalls. Early rains saturated the soil over more than a 300,000 square miles of the upper watershed, greatly reducing infiltration and leaving soils with little or no storage capacity. As rains continued, surface depressions, wetlands, ponds, ditches, and farm fields filled with overland flow and rainwater. With no remaining capacity to hold water, additional rainfall was forced from the land into tributary channels and thence to the
Mississippi River The Mississippi River is the second-longest river and chief river of the second-largest drainage system in North America, second only to the Hudson Bay drainage system. From its traditional source of Lake Itasca in northern Minnesota, it f ...
. For more than a month, the total load of water from hundreds of tributaries exceeded the Mississippi’s channel capacity, causing it to spill over its banks onto adjacent floodplains. Where the flood waters were artificially constricted by an engineered channel bordered by constructed levees and unable to spill onto large section of floodplain, the flood levels forced even higher.


See also

*
Hydrological modelling A hydrologic model is a simplification of a real-world system (e.g., surface water, soil water, wetland, groundwater, estuary) that aids in understanding, predicting, and managing water resources. Both the flow and quality of water are commonly stud ...
*
List of rivers by discharge This is a list of rivers by their average discharge (that is, their water flow rate). Here, only those rivers whose discharge is more than are shown, as this list does not concern itself with rivers having a water flow rate of less than . For c ...
*
Losing stream A losing stream, disappearing stream, influent stream or sinking river is a stream or river that loses water as it flows downstream. The water infiltrates into the ground recharging the local groundwater, because the water table is below the bo ...
*
Perennial stream A perennial stream is a stream that has continuous flow of surface water throughout the year in at least parts of its catchment during seasons of normal rainfall, Water Supply Paper 494. as opposed to intermittent river, one whose flow is interm ...
*
Runoff model (reservoir) A runoff model is a mathematical model describing the rainfall–runoff relations of a rainfall ''catchment area'', drainage basin or ''watershed''. More precisely, it produces a surface runoff hydrograph in response to a rainfall event, repres ...
*
Stream bed A stream bed or streambed is the bottom of a stream or river (bathymetry) or the physical confine of the normal water flow (Channel (geography), channel). The lateral confines or channel margins are known as the stream Bank (geography), banks ...
*
Water resources Water resources are natural resources of water that are potentially useful for humans, for example as a source of drinking water supply or irrigation water. 97% of the water on the Earth is salt water and only three percent is fresh water; slight ...
*
Open-channel flow In fluid mechanics and hydraulics, open-channel flow is a type of liquid flow within a conduit with a free surface, known as a channel. The other type of flow within a conduit is pipe flow. These two types of flow are similar in many ways but diff ...


References

*USGS, Atlanta, GA
"The Water Cycle: Streamflow."
2 August 2010. {{refend Hydrology