Streamflow, which is also known as channel runoff, refers to the flow of water in natural watercourses such as streams and rivers. 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.
A given watercourse has a maximum streamflow rate that can be accommodated by the channel, and which 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.
Since the streamflow can be calculated, watercourses that are prone to flooding can be modified to reduce the likelihood of flooding, or development of lands in the vulnerable areas can be restricted.
Historical Background and Scientific Foundations
Measurement of streamflow in a small watercourse can be accomplished by releasing a floating object from a designated spot and timing how long it takes the object to travel to another designated spot farther downstream. By knowing the distance between the two points and the travel time, the flow can be calculated (typically as cubic meters per second). The result of at least three determinations is known as a hydrograph.
The measurements should be conducted in the middle of the watercourse, since this is where the water is deepest. The flow toward either side can be slower, due to the increased friction of the shallower water with the bottom.
The result can be made even more realistic of the actual streamflow by factoring in the nature of the bottom of the watercourse. A rough bottom will produce a more turbulent waterflow, as the water will flow around and over the rocks and stones present. This will slow the streamflow from the value calculated using the speed of the floating object. Generally, the streamflow value obtained from midstream is multiplied by 0.8 to approximate the actual flow rate.
A smoother bottom will not impede flow as much. For such watercourses, the calculated mid-stream flow rate is multiplied by 0.9 to obtain the estimate of the actual flow.
Watercourses that are wider and deeper are more complicated to measure. For these, the width, depth, and waterflow must be measured at many spots across the width of the watercourse and at multiple depths. The measurements are combined to yield an average flow at the average depth.
For these measurements, a simple floating object is insufficient. Rather, a piece of equipment called a current meter can be used. The meter consists of a probe attached to a torpedo-shaped object. The object has fins at one end that positions the object in the path of the current. Spinning cups on the probe measure the velocity of the passing water, which is recorded electronically. Streamflow is not constant. This is especially evident after a heavy rainstorm or during the springtime snowmelt. A stream or river that is gently flowing one day can contain a thundering flow of water only hours later, as the excess water enters the watercourse directly or from surface runoff. The change is especially evident in larger rivers that are receiving the flow of water from
WORDS TO KNOW
EROSION: The wearing away of the soil or rock over time.
RUNOFF: Water that falls as precipitation and then runs over the surface of the land rather than sinking into the ground.
SPRING: The emergence of an aquifer at the surface, which produces a flow of water.
WATERSHED: The expanse of terrain from which water flows into a wetland, water body, or stream.
other watercourses in a watershed, since the combined flow of the various watercourses is combined into the larger watercourse.
By monitoring streamflow, the capacity of the watercourse can be determined at various points. Areas that are narrower or have bends may be more susceptible to flooding than other areas. All this is valuable information, since warnings can be issued when conditions are such that the capacity of the watercourse is likely to be exceeded. Susceptible areas of the watercourse may be altered or barriers installed to lessen the chances of flooding.
Impacts and Issues
Streamflow is a vital part of the water cycle, where water is cycled between Earth’s surface and the atmosphere, and is vital for the movement of water within a watershed.
Knowledge of the streamflow of a particular watercourse is also important in determining how best to use the land that borders the watercourse. For example, regions that are determined to be susceptible to flooding should not be used for residential, industrial, or commercial development. Use of such regions as parkland, for example, allows the land to function as a floodplain when necessary.
As well, the knowledge of streamflow is important in determining environmentally acceptable activities that are permitted in the lands bordering the river. For example, it would be unwise to permit the release of industrial effluent into a watercourse with a very slow flow, since any toxic compounds in the effluent would have more opportunity to affect life in the water than in a watercourse with a high flow rate.
Localized manipulation of streamflow is exploited as a means of generating power. By damming a watercourse and releasing the water at a controlled rate, the artificially created higher streamflow is used to generate hydroelectricity.
Bray, R. N. Environmental Aspects of Dredging. New York: Taylor & Francis, 2008.
Chiras, Daniel D., John P. Reganold, and Oliver S. Owen. Natural Resource Conservation: Management for a Sustainable Future. New York: Prentice-Hall, 2004.
Grover, Velma I. Water: Global Common and Global Problems. Enfield, NH: Science Publishers, 2006.