Streamflow, or channel runoff, is the flow of water in streams, rivers, and other channels, and is a major element of the water cycle. It is one component of the runoff of water from the land to waterbodies, the other component being surface runoff. Water flowing in channels comes from surface runoff from adjacent hillslopes, from groundwater flow out of the ground, and from water discharged from pipes. The discharge of water flowing in a channel is measured using stream gauges or can be estimated by the Manning equation. The record of flow over time is called a hydrograph. Flooding occurs when the volume of water exceeds the capacity of the channel.
Role in the water cycle 
Streamflow is the main mechanism by which water moves from the land to the oceans or to basins of interior drainage.
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. For purposes that do not require a continuous measurement of stream flow over time, current meters or acoustic Doppler velocity profilers can be used. For small streams — a few meters wide or smaller — weirs may be installed. 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.
Flow monitoring in the United States 
In the United States, streamflow gauges are funded primarily from state and local government funds. In Fiscal Year 2008 the United States Geological Survey (USGS) provided 35 percent of the funding for everyday operation and maintenance of gauges.
Relationship to the environment 
Relationship to society 
Streamflow confers on society both benefits and hazards. Runoff downstream is a means to collect water for storage in dams for power generation of water abstraction. The flow of water assists transport downstream. When the flow of water exceeds the capacity of the channel, flooding occurs.
See also 
- Discharge (hydrology)
- Drainage basin or watershed
- Drainage network
- Hydrological modelling
- Losing stream
- Runoff model (reservoir)
- Surface runoff
- Stream bed
- Water resources
- R.G. Wetzel, G.E. Likens: Limnological Analyses, pp. 62–63.
- U.S. Forest Service. "10. Measuring Discharge."
- Delaware River Basin Commission. West Trenton, NJ. "Who Pays for the Maintenance of Gaging-Stations?" 2009-04-30.
- USGS, Atlanta, GA. "The Water Cycle: Streamflow." 2010-08-02.
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