Streamflow, or channel runoff, is the flow of water in streams and other channels, and is a major element of the water cycle. It is one runoff component, the movement 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 Streams play a critical role in the hydrologic cycle 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 and food resources. 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. 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 is the moisture falling directly on the water surface, and in most streams, it adds very little to discharge. Groundwater 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 is water that infiltrates the soil and then moves laterally to the stream channel in the zone above the water table. Much of this water is transmitted within the soil, some of it moving within the horizons. Next to baseflow, 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 is usually a major source of streamflow. Overland flow is a stormwater 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 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 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 retention 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 Climate Change
Increase/decrease in precipitation Air temperature change altered streamflow variability reduced snowpack increased droughts and floods Increases/decreases in precipitation contribute significantly to changes in streamflow. Researchers from the DRI examined over 500 watersheds across the USA and found increased winter temperatures are causing more extreme fluctuations in streamflow. Data show that watersheds with lots of snow are receiving more precipitation as rain than previously. As a result streams now have more water coming in quick surges from rainstorms, rather than the slow flow of melting snow. These fluctuations could also be attributed to faster snowpack melting due to higher temperatures. From a groundwater perspective, higher evaporation and evapotranspiration rates reduce soil moisture and groundwater recharge, lowering baseflow (the sustained part of streamflow from groundwater).
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 (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 meters or acoustic Doppler velocity profilers can be used. For small streams—a few meters wide or smaller—weirs may be installed.
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