Hobbies And Interests
Measuring Discharge Volume
The volume of water that flows through a river cross-section in a given time is the river's flow rate, or discharge volume. It is measured in cubic meters or cubic feet per second. Water experts, called hydrologists, measure how fast the water is moving at the surface. From that, they estimate average water velocity. Average water velocity is affected by how the river slopes, twists and turns, as well as by the roughness of the riverbed. Then hydrologists calculate the width and depth of the river at the point where they're measuring water speed. Those factors, put together, give discharge volume.
Site Selection
For best results in measuring river discharge volume, the World Health Organization (WHO) recommends that hydrologists should set up a permanent gauging station. The station should be built at a point where the river is straight just above and below the station. The riverbed should be "regular and stable," the stream not overflow its banks, and the waterway not clogged with aquatic growth. A bridge is a good site for a measuring station, since instruments to gauge river speed and changing depth can be attached to the bridge. If the waterway is shallow enough, the investigator can wade in with the measuring device himself. If no station can be built at that site, hydrologists can throw a float into the river and time its passage downstream.
Sedimentary Journey
Rivers don't just discharge water. They also discharge sediment, which is carried downstream as land erodes. Measuring this discharge allows scientists to calculate how quickly the land upstream is wearing down. It also shows how fast the delta downstream at the river's mouth is building up. Measuring the river water discharge rate at regular intervals gives a good approximation of the volume of sediment that leaves a river's drainage basin. It is also important to measure differing discharge volumes during floods, when most sediment movement occurs.
Some Statistics
According to the U.S. Geological Survey, the longest U.S. river, the Mississippi, discharges 593,000 cubic feet of water at its mouth, in Louisiana, every second. Every 1,000 cubic feet of water passing every second represents 646 million gallons of water per day. Natural and man-made changes can have a big effect on a river's discharge volume. For instance, the Colorado River, dammed and diverted to serve the parched Southwest, has had its average discharge volume cut by more than three-fourths from the days when it ran wild. However, a high snowmelt or record rains can push that number back up toward its historic levels.