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.
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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:
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.[3]
Runoff of water in channels is responsible for transport of sediment, nutrients, and pollution downstream.
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.