Geological Survey (USGS) does quite a lot of work across the country measuring how much sediment is transported by streams. Geological Survey (USGS) hydrographer is collecting a suspended-sediment water sample from the Little Colorado River, a kilometer upstream from the Little Colorado River, Grand Canyon, Arizona, USA. The very brown water here indicates the presence of a lot of fine dirt particles and the turbidity of this water is very high. Reservoirs slowly fill up with sediment and mud, eventually making them unusable for their intended purposes. 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. 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. Sediment in rivers can also shorten the lifespan of dams and reservoirs. 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. The fertile floodplains of the Nile in Egypt and of the Mississippi River in the United States have flooding rivers to thank for their excellent soils. 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 sediment may build up on the bottom or it may get picked up and suspended again by swift-moving water to move further downstream.
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). If you leave your glass in a quiet spot for a while the sediment will start to settle to the bottom of the glass. If you scoop up some muddy river water in a glass you are viewing the suspended sediment in the water.
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. 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. Fast-moving water can pick up, suspend, and move larger particles more easily than slow-moving waters. 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 larger that are moved by the water are called sediment. The large amount of suspended sediment can harm water quality of not only the tributary but also of the receiving river. If preventative measures are not taken on construction sites where runoff flows into a nearby stream or creek, results like this can happen.
Sediment-laden water from a tributary, where development is probably taking place, entering the clearer Chattahoochee River near Atlanta, Georgia.