Alluvial systems
Alluvial systems
An alluvial system consists of sediments eroded, transported, and deposited by water flowing in rivers or streams. The sediments, known as alluvium, can range from clay-sized particles less than 0.002 mm (.00008 in) in diameter to boulders, which are defined as rocks greater than 256 mm (10 in) in diameter, depending on their source and the sediment transport capacity of streams in the system. The term alluvial is
closely related to the term fluvial, which refers to flowing water. Thus, alluvial systems are the result of fluvial processes.
Modern alluvial systems can create flat and fertile valley bottoms that are attractive for farming because of their rich soils, which are replenished during frequent floods. The same floods that replenish soils, though, can become hazardous when homes are built on floodplains. Ancient alluvial systems that now lie below Earth’s surface can be exceptionally good aquifers and petroleum reservoirs.
Alluvium
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 bed-load 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 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.
Commmon components
The channel is the depression through which water flows from the head to the mouth of a stream. Some channels follow sinuous, or meandering, paths. Erosion occurs along the outside edge of meanders, where water velocity is the greatest, and deposition occurs along the inside edge of meanders, where water velocity is the lowest. The resulting depositional feature is known as a point bar, and alluvial channel deposits are formed by the constant migration of meanders through geologic time. Other channels are braided, meaning that they are composed of two or more interconnected low-sinuosity channels. Braided stream channels form when the sediment load is large compared to the sediment transport capacity of a stream. Braided streams can occur naturally in areas of high sediment supply, for example near the snout of a glacier (the end of a glacier) or where the stream gradient abruptly decreases. A change from a meandering stream to a braided stream over a few years or decades, however, can indicate an undesirable increase in sediment supply as the result of activities such as cattle grazing or logging.
During times of high discharge, for example shortly after heavy rainstorms or spring snowmelt, streams can rise above their banks and flood surrounding areas. The low-lying and flood-prone areas adjacent to streams are known as floodplains. The velocity of water flowing across a floodplain is much lower than that in the adjacent channel, which causes suspended sediment to be deposited on floodplain. Coarse sediment is deposited close to the channel and forms natural levees that help to control future floods, but silt and clay can be carried to the far reaches of the floodplain. Thus, alluvial systems generally consist of both coarse-grained channel deposits representing bedload and fine-grained floodplain (also known as overbank) deposits.
Meanders can be abandoned, particularly when they become extremely sinuous, to form crescent-shaped oxbow lakes within the flood plain. Abandonment occurs when it becomes more efficient for the stream to transport its sediment load by cutting a short new channel, thereby locally increasing its gradient, than by flowing through a long meander.
Another common feature of alluvial systems is the stream terrace. A stream terrace is simply a floodplain that was abandoned when the stream incised to a lower level due to a change in base level. Careful observation can often reveal several generations of step-like terraces, each of which represents the elevation of an abandoned flood plain, along a stream valley.
Coastal alluvial plains
Coastal alluvial plains form when streams transporting sediment from mountainous areas reach low elevations and deposit a large proportion of their sediment load. This occurs due to a decrease in stream gradient, which reduces sediment transport capacity. The extent of coastal alluvial plains is controlled in large part by sea level, and alluvium deposited during previous times of low sea level (for example, during glacial epochs) may now lay tens or hundreds of meters below sea level.
Alluvial fans
Alluvial fans form where high-gradient mountain streams flow into valleys or onto plains and deposit their sediment load. Such is the case along the foot of the Panamint Mountains bordering Death Valley in California. As with overbank floodplain deposits, the
KEY TERMS
Bajada— A feature produced when adjacent alluvial fans overlap or coalesce and form a continuous deposit at the foot of a mountain range. Bajadas are common features in arid to semi-arid regions of the American West and Southwest.
Base level— The lowest elevation to which a fluvial system grades or adjusts itself. Local base levels can be lakes or larger rivers. The global base level is sea level, which changes through geologic time.
Bedload— The portion of sediment that is transported by rolling, skipping, and hopping along the stream bed at any given time because it is too heavy to be lifted by flowing stream water. It stands in contrast to suspended load.
Discharge— The volume of water flowing across an imaginary vertical plane perpendicular to the stream channel per unit of time. In the United States, it is customary to express stream discharge in units of cubic feet per second.
Gradient— The slope of a stream channel, measured in terms of the change in elevation per unit of channel length. Stream gradients can be expressed as percentages or using dimensionless terms such as meters of elevation change per kilometer of channel length.
Sinuosity— The degree of curvature of a stream channel as viewed from above. Highly sinuous streams contain many curves or meanders along their lengths.
Suspended load— Sediment particles transported within flowing stream water at any given time, as opposed to bedload. Suspended load is responsible for the muddiness or turbidity of river water.
coarsest sediment is generally deposited closest to the mountains, and finer sediment can be carried many miles. Geologists often refer to the sediment deposited near the mountains as proximal and the sediment deposited far from the mountains as distal. Alluvial fans typically form three-dimensional cones that resemble folding fans when viewed from above, hence their name. The main stream branches out into many channels that distribute sediment across the alluvial fan. Alluvial fans formed by streams that drain large watersheds tend to be larger than those that drain small watersheds. They can grow in size until they begin to merge with fans created by neighboring streams, at which point they coalesce into a broad sloping surface known as a bajada.
Deltas
Deltas are formed at the mouths of streams that flow into lakes or oceans. They are fan-like deposits similar to alluvial fans, but located in the water rather than on dry land. Like alluvial fans, coarse sediments are deposited close to shore and fine-grained sediment is carried farther out to sea. The Mississippi River has formed the most prominent example of a delta within the United States. Other well-known examples are the Nile Delta of North Africa and the Amazon Delta of South America. When Aristotle (384–322 BC) observed the Nile Delta, he recognized it was shaped like the Greek letter delta, hence the name. Most deltas clog their channels with sediment and so must eventually abandon them. If the river then flows to the sea along a significantly different path, the delta will be abandoned and a new delta lobe will form. This process, known as delta switching, helps build the coastline outward.
See also Continental margin; Continental shelf; Earth science; Global climate; Hydrologic cycle; Hydrology; Land use; Landform; Sediment and sedimentation; Sedimentary environment; Sedimentary rock.
Resources
BOOKS
Brierley, Gary J. Geomorphology and River Management: Applications of the River Styles Framework. Malden, MA: Blackwell Publishing, 2005.
Gordon, Nancy D., et al. Stream Hydrology. Chichester, UK, and Hoboken, NJ: Wiley, 2004.
Nagle, Garrett. Rivers and Water Management. London, UK: Hodder & Stoughton, 2003.
Richards, Keith R. Rivers: Form and Process in Alluvial Channels. Caldwell, NJ: Blackburn Press, 2004.
Schaetzl, Randall J. Soils: Genesis and Geomorphology. Cambridge, UK, and New York: Cambridge University Press, 2005.
OTHER
“Glossary of Hydrologic Terms.” National Weather Service, National Oceanic and Atmospheric Administration.;http://www.srh.noaa.gov/wgrfc/resources/glossary/default.html; (accessed November 9, 2006).
“Glossary of Hydrologic Terms.” United States Geologic
Service. (accessed November 9, 2006). ;http://or.water.usgs.gov/projs_dir/willgw/glossary.html>.
Pidwirny, Michael. “Fluvial Landforms.” University of British Columbia Okanagan. ;http://www.physicalgeography.net/fundamentals/10z.html> (accessed November 9, 2006).
William C. Haneberg