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.

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.

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.