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 ivers are in the business of moving two things: water and sediment. We've seen how rivers gather water, but where does the sediment come from? Wind occasionally blows some sand and dust into a river, but far more sediment is picked up by water flowing overland. Rock exposed alongside a river can weather, breaking down into smaller particles that are carried away by moving water. When a river undermines its banks, entire shelves of rock and mud collapse into the current. In flood, a river scoops up sand that has accumulated along its bed during quieter times, and flushes that sediment downstream. The range of sediment concentrations can be extreme. High mountain streams may be crystal clear most of the year. In its lower reaches during a flood, the muddy Mississippi can deliver several million tons of sediment each day to the Gulf of Mexico. Sediments are carried downstream by different processes depending on their size. Throughout the world, much of the earth's surface consists of easily eroded rock known as shale which, when wet, quickly disintegrates into tiny particles of clay. Stirred by a flood, this suspended sediment gives a river its muddy character. Sand grains, larger than silt or clay, bounce along a riverbed, hopping downstream in a repeated series of short jumps called saltation. The largest particles, such as cobbles and boulders, usually rest on a stream's bottom, rolling only when the river is in high spirits. |