The USGS Water Science School
Sediment and Suspended Sediment
Sediment-laden water from a tributary, where development is probably taking place, entering the clearer Chattahoochee River near Atlanta, Georgia
Storms, of course, deliver large amounts of water to a river, but did you know they also bring along lots of eroded soil and debris from the surrounding landscape? Rocks as small as tiny clay particles and as large as boulders moved by the water are called sediment. Fast-moving water can pick up, suspend, and move larger particles more easily than slow-moving waters. This is why rivers are more muddy-looking during storms—they are carrying a LOT more sediment than they carry during a low-flow period. In fact, so much sediment is carried during storms that over one-half of all the sediment moved during a year might be transported during a single storm period.
If you scoop up some muddy river water in a glass you are viewing the suspended sediment in the water. If you leave your glass in a quiet spot for a while the sediment will start to settle to the bottom of the glass. The same thing happens in rivers in spots where the water is not moving so quickly—much of the suspended sediment falls to the stream bed to become bottom sediment (yes, mud). The sediment may build up on the bottom or it may get picked up and suspended again by swift-moving water to move further downstream.
So what does this have to do with people? On the positive side, sediment deposited on the banks and flood plains of a river is often mineral-rich and makes excellent farmland. The fertile floodplains of the Nile in Egypt and of the Mississippi River in the United States have flooding rivers to thank fo rtheir excellent soils. On the negative side, when rivers flood, they leave behind many tons of wet, sticky, heavy, and smelly mud—not something you would want in your basement.
Sediment in rivers can also shorten the lifespan of dams and reservoirs. When a river is dammed and a reservoir is created, the sediments that used to flow along with the relatively fast-moving river water are, instead, deposited in the reservoir. This happens because the river water flowing through the reservoir moves too slowly to keep sediment suspended -- the sediment settles to the bottom of the reservoir. Reservoirs slowly fill up with sediment and mud, eventually making them unusable for their intended purposes.
Sediment-data collection in the Little Colorado River a kilometer upstream from the Colorado River, Grand Canyon, Arizona
The U.S. Geological Survey (USGS) does quite a lot of work across the country measuring how much sediment is transported by streams. To do this, both the amount of water flowing past a site (streamflow or flow) and the amount of sediment in that water (sediment concentration) must be measured. Both streamflow and sediment concentration are continually changing.
Streamflow is measured by making a discharge measurement. Suspended sediment, the kind of sediment that is moved in the water itself, is measured by collecting bottles of water and sending them to a lab to determine the concentration. Because the amount of sediment a river can transport changes over time, hydrologists take measurements and samples as streamflow goes up and down during a storm. Once we know how much water is flowing and the amount of sediment in the water at different flow conditions, we can compute the tonnage of sediment that moves past the measurement site during a day, during the storm, and even during the whole year.
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