Water Science for Schools
Introduction 
1: Measuring stage 2: Discharge measurement 3: Stage-discharge relation 4: Converting stage to streamflow
Streamgages continuously measure stage, as stated in the “Measuring Stage” section. This continuous record of stage is translated to river discharge by applying the stage-discharge relation (also called rating). Stage-discharge relations are developed for streamgages by physically measuring the flow of the river with a mechanical current meter or ADCP at a wide range of stages; for each measurement of discharge there is a corresponding measurement of stage. The USGS makes discharge measurements at most streamgages every 6 to 8 weeks, ensuring that the range of stage and flows at the streamgage are measured regularly. Special effort is made to measure extremely high and low stages and flows because these measurements occur less frequently. An example of a stage-discharge relation is shown in the diagram below. The stage-discharge relation depends upon the shape, size, slope, and roughness of the channel at the streamgage and is different for every streamgage.
The development of an accurate stage-discharge relation requires numerous discharge measurements at all ranges of stage and streamflow. In addition, these relations must be continually checked against on-going discharge measurements because stream channels are constantly changing. Changes in stream channels are often caused by erosion or deposition of streambed materials, seasonal vegetation growth, debris, or ice. An example of how erosion in a stream channel increases a cross-sectional area for the water, allowing the river to have a greater discharge with no change in stage, is shown in the diagram below. New discharge measurements plotted on an existing stage-discharge relation graph would show this, and the rating could be adjusted to allow the correct discharge to be estimated for the measured stage.
Sources and more information
The water cycle: Streamflow
U.S. Department of the Interior |
U.S. Geological Survey
URL: http://ga.water.usgs.gov/edu/streamflow3.html
Page Contact Information: Howard Perlman
Page Last Modified: Thursday, 22-Dec-2011 10:25:43 EST
