As you can tell by looking at Dirty Drippy emerging from the river
in this picture, rivers and streams are hardly ever crystal clear. As
the rivers move they are carrying soil, sand, and sediment along with
them. The sediments that rivers transport actually play quite an
important role in shaping the environment and even in our own
lives.
When it rains, soil and debris from the surrounding land are eroded
and washed into streams. From there, sediment particles from as small
as clay to as large as boulders flow along with the water. 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 well over one-half of all
the sediment moved during a year might be transported during a single
storm period.
So what does this have to do with people? On the plus side, sediment deposited on the banks and flood plains of a river is often mineral-rich and makes excellent farmland. The Nile in Egypt and the Mississippi River here in the United States are good examples. 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. You may recall the disastrous effects of the Midwest flooding of 1995 and 1997. Sediments can also harm 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.
The U.S. Geological Survey does quite a lot of work measuring how much
sediment is transported by streams across the country. 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.
A river discharge measurement is performed to measure
streamflow. As streamflow goes up and down during a storm,
hydrologists take measurements of how much sediment is in the water at
different streamflows. 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.
• Suspended sediment, USGS Office of Surface Water.
• The USGS California office has a Web page about
floods and flood plains