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Water Flow

What does this mean?

Most of us perceive water flow as the speed or rate at which water appears to be moving, referred to as "current". Hydrologists and other environmental scientists use a more exact measure for rivers and streams, called "discharge" or "flow". It is defined as the volume of water moving past a particular point during a given time period. Discharge or flow (either term is acceptable) applies to rivers and streams and is reported as cubic feet per second (cfs) or cubic meters per second (cms).

The data presented on the Water Atlas represent how the current discharge of a river in cubic feet per second (cfs) compares to the historical average. Tables provide discharge values summarized for the entire river, and from individual stations along the river. The latter is more meaningful, since water flow/discharge can vary considerably in different parts of a stream, particularly in major river systems.

History: Measuring water flow began at the turn of the 19th century in response to the demand for water as large populations centers developed around streams. As in current times, these communities were using, and over-using, vast amounts of water for drinking, industry, transportation and waste disposal. Flow gages are also used in predicting flood events, protecting lives and property. The first federally-funded gaging station (flow monitoring station) was established on the Rio Grande in 1889, and the first flow gaging network (seven stations) was established in Kansas in 1895. Since this time, hundreds of gaging stations have been established by federal, state, county and municipal agencies, and by private organizations for purposes of water resource planning, flood control, irrigation, environmental restoration and navigation. In fact, stream gaging has become the backbone of virtually every water resource and water quality management program worldwide.


How are the data collected? (Methods)

Discharge measurements are made using a variety of methods and technologies. Handheld sensors can be used to measure flow while wading across streams between specific points on the banks. In many watersheds, permanent stations are constructed on stream banks or over streams such that sensors can be deployed permanently and discharge can be measured and monitored as often as desired, including via remote sensing methods. Scientists refer to this continuous monitoring of flow as "gaging" and the devices used to make those measurements are referred to as gages or streamgages.

Although methods, techniques, and technologies vary, all gaging first involves developing a clear understanding of the physical features of the stream or canal such as its width, water depth, the amount and degree of meandering, the slope and "roughness" (takes into account the composition of the bottom and how it affects water movement) of the channel, locations of log jams or dams, and the location of confluences with other waterbodies. If any historic data exist, these are studied to learn more about important factors such as temperature extremes, floods and drought, all of which are useful for determining where to place a gaging station and how it should be equipped.


Calculations

Stream discharge cannot be measured directly. It is computed from other direct measurements: stream depth, stream width, and streamflow velocity. This is generally done by multiplying the cross-sectional area of water in the stream channel by the average velocity of the water in that cross section.

Flow (or discharge) at one point in a stream can be calculated using the following formula:

Flow = water velocity × channel width × water depth

Comparing discharge measurements from season to season and year to year provides valuable insight regarding water quantity trends in streams and their watersheds. When combined with water quality and land use data, for example, scientists are able to determine the positive and negative impacts of various activities in the watershed.


Caveats and Limitations

Flow data are most valuable when collected over long periods of time and at the same locations, and when analyzed with extensive knowledge of land use and meteorological conditions. Accurate representation of flow regimes can only be accomplished with frequent measurements, over long periods of time, using accurate, regularly calibrated gaging instruments.


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