Alluvial Systems

Alluvium is the product of sediment erosion, transportation, and deposition. Therefore, its nature is controlled by the sediment supply and sediment transport capacity of streams in the watersheds from which it is derived. Regions subjected to high rates of tectonic uplift or tropical climates can supply large amounts of sediments to fluvial systems because rocks weather rapidly under those conditions. Tectonically stable, arid, and cold regions, in contrast, generally produce sediments at much lower rates. Global sea level changes influence alluvial processes in much the same way as tectonic uplift, controlling the base level to which streams erode.

The type of bedrock underlying a watershed also exerts an important influence on the nature of the alluvium deposited downstream. A drainage basin underlain by easily eroded sandstone may produce only sand and silt, whereas one underlain by hard metamorphic rocks may produce cobbles and boulders. Geologists often study sedimentary rocks that were deposited in alluvial systems in order to learn about the climate, tectonics, and bedrock geology of ancient mountain ranges that supplied the sediment.

Once sediment is produced by the physical and chemical weathering of rocks, it must be transported to an area in which deposition, also known as aggradation or alluviation, is possible. Although sediment transport is a complicated process, it is well known that sediment transport capacity is exponentially proportional to the stream discharge. Discharge is in turn controlled by precipitation, channel shape, channel roughness, and stream gradient. Graded streams have adjusted their longitudinal profiles (by erosion and deposition at different points) in order to transport the volume of sediment supplied to them.

Sediment particles that are small enough to be lifted and carried within the flowing water are known as the suspended load, whereas larger particles that roll, bounce, or skip along the streambed are known as the bedload. The specific sizes of sediment transported as suspended load and bedload will change as the stream discharge changes in space and time. Therefore, a sediment particle may be part of the bedload during times of discharge and part of the suspended load during times of high discharge such as floods. Sediment that is small enough to remain constantly suspended is often referred to as the wash load. Although a large river may be capable of transporting large boulders, its supply may be limited by the ability of watersheds boulders to produce or tributaries to deliver them to the river. In many cases, boulders that exceed the sediment transport capacity of tributary streams are supplied to rivers by landslides, rockfalls, and debris flows.

Alluvium is deposited during repeated cycles of valley incision and alluviation. Valley incision generally occurs during times of rapid tectonic uplift (when sediment transport capacity exceeds sediment supply) or dry climates (when little sediment is produced). Alluviation, in contrast, is more likely to occur during times of tectonic quiescence or wet climatic conditions. Successive cycles of incision and alluviation can produce a series of stream terraces, which are the remnants of abandoned flood plains and appear to form steps along valley walls.