Alluvial Systems

Alluvial systems include a variety of different depositional systems, excluding deltas, that form from the activity of rivers and streams. Much alluvial sediment is deposited when rivers top their banks and flood the surrounding countryside. Buried alluvial sediments may be important water-bearing reservoirs or petroleum-containing strata.

Sediment

Deposits of silt, sand, and gravel produced by the activity of rivers and streams are called alluvial sediments. Sediment in rivers is moved primarily as either suspended load or bed load. The suspended load is the finest portion of sediment, composed of silt and clay particles, and is carried suspended within the flow itself by fluid turbulence. Material moved along the bottom of the river by rolling, sliding, and bouncing (saltation) is called the bed load, and it makes up the coarse fraction of a river’s sediment. Rivers can be divided into four categories based on their morphology: braided, meandering, anastomosing, and straight. Straight rivers are rare, usually appearing only as portions of one of the other river types, and anastomosing rivers can be considered a special type of meandering river.

Several criteria are used to characterize alluvial systems. They include grain size, dominant mode of sediment transport (suspended load versus bed load), and migrational pattern of the river channel. Alluvial sediments can be broadly divided into three interrelated depositional settings: braided rivers, alluvial fans, and meandering rivers.

Braided Rivers

Braided rivers have low sinuosity, which is defined as the ratio of the length of the river channel to the down-valley distance. They are characterized by relatively coarse-grained sediment transported as bed load. Fine-grained sediments make up a minor portion of the deposits. The main channel is internally divided into many subchannels and bars, which give the river a braided pattern. River bars are ridgelike accumulations of sand and gravel formed in the channel or along the banks, where deposition is induced by a decrease in velocity. Transverse bars are flat-topped ridges, oriented transversely to the flow, that grow by down-current additions and migration of sediment. Longitudinal bars are midchannel sand and gravel accumulations oriented with their long axes roughly parallel to current flow. During low-water stages, the braided pattern is very apparent, and water occupies only the subchannels. It is only during high-water stages that the entire braided channel has water in it, at which time the braided appearance may no longer be evident. The bars that occur in these rivers form as a result of high sediment loads and fluctuations in river discharge.

Braided rivers form in regions where sediment is abundant, water discharge fluctuates (but may be high), and, usually, vegetation is sparse. Some braided rivers and streams have flowing water in them sporadically, with long periods of dryness in between. These streams are called ephemeral, or intermittent. Braided rivers tend to have relatively high gradients. As such, they commonly occur as the upper reach of a river that may become a meandering river downstream as the sediment’s grain size and the gradient both decrease.

Alluvial Fans

Alluvial fans are deposits that accumulate at the base of a mountain slope. There, mountain streams encounter relatively flat terrain and lose much of their energy, subsequently depositing the sediment that they were moving. The effect is similar to that of delta formation, which occurs as a flowing stream enters a body of relatively still water and loses the energetic motion necessary to maintain its suspended sediment load. This type of stream shifts the position of its channel over time, as sediment continues to be deposited, and builds a fan-shaped accumulation of debris that is coarse-grained near the mountain front and becomes progressively finer-grained away from the highland. Alluvial fans are best developed and observed in arid or semiarid climates, where vegetation is sparse and water flow is intermittent. Large quantities of sediment may be moved during short-term flash-flood events. There are also humid-climate fans, such as the enormous Kosi fan in Nepal, which measures 150 kilometers in its longest dimension.

Both arid and humid fans are built at least in part by deposition in a braided stream environment. Stream discharge, and therefore sediment deposition, is discontinuous on arid fans. Braided streams may operate over the fan’s entire surface or predominantly on the outer regions, away from the region where the stream leaves the confines of the mountain valley. Arid fans are also built by deposition from mudflows and debris flows. These flows differ from braided stream flows in that they contain a lower proportion of water and a much higher proportion of debris. Humid fans have braided stream systems operating continuously on their surface. Their overall deposits are similar to braided stream deposits formed in other sedimentary environments.

Meandering Rivers

Meandering rivers have a greater sinuosity than braided rivers and are usually confined to a single channel. They have a lower gradient and therefore are typically located downstream from braided rivers. Sediment in meandering rivers is moved mostly as fine-grained suspended load. Several different types of sedimentary deposit result from the activity of meandering rivers. The coarsest material available to the river is moved and deposited within the deepest part of the channel. These gravelly deposits are thin and discontinuous. Point bars develop on the inside curve of a meander bend and are a major site of sand deposition, although silt and gravel may also be components of point-bar deposits. Deposition takes place on point bars because of flow conditions in the river as water travels around the bend. Erosion takes place on the bank opposite the point bar, and in this way, meanders migrate. When a river floods and tops its banks, water and finer-grained sediment spill out of the channel and onto the surrounding valley floor, which is called the floodplain. On the bank directly adjacent to the river channel, large amounts of sediment are deposited to form natural levees, which are elongated narrow ridges parallel to the channel. Levees help to confine the river but are still topped during major floods. As the river spreads out across the floodplain, silt and clay are deposited.

Meandering rivers are constantly shifting their location within the river valley. In this way, very thick alluvial deposits may accumulate through time. New channels may be created between meanders such that old meander bends are cut off and isolated. These isolated meander bends are termed “oxbow lakes” and tend to fill quickly with sediment.

A special type of meandering river is an anastomosing river. It is characterized by a system of channels that do not migrate as much as meandering river channels and that are separated by large, permanent islands.

Transport and Deposition

Alluvial sediments require running water for transport and deposition. This water may be available year-round, such that rivers and streams are constantly active. Sporadic stream discharge produces alluvial sediments in arid and semiarid environments. Alluvial sediments are also associated with glaciers. Large amounts of sediment with a wide range in grain size are deposited directly by glaciers. Streams fed by glacial meltwaters are important and effective agents of transport and deposition of this sediment. Most streams associated with glaciers are bed load streams and therefore have a braided pattern. Their sediment is usually quite coarse-grained.

Alluvial systems form broad, interconnected networks of drainage that feed water and sediment from highlands or mountainous regions to lowlands and eventually to the sea. These drainages form recognizable patterns that are controlled by the type of rock; by the type of deformation, if any, that the earth’s crust in the region has undergone; and by the region’s climate. As river systems age, the landscape changes and evolves. In this way, the surface of the land is sculpted by rivers, in concert with other surface processes.

Study of Alluvial Systems

The study of alluvial systems can be divided roughly into two categories: the study of modern river systems and the study of sedimentary rocks interpreted to have formed in some type of alluvial system. The study of modern alluvial processes and the sedimentary deposits that they generate is crucial to the understanding of such deposits in the geological record of rock strata. By understanding modern rivers—the ways in which sediment is moved and deposited, the changes through time in channel shape and location, and the characteristics of the deposits in relation to specific physical conditions—geologists can begin to interpret ancient alluvial sediments.

Modern alluvial systems are studied in a number of ways. It is important to know as much as possible about the flow of water itself, so measurements are made of flow velocity, depth, and width. The channel shape and configuration are also measured. Samples of river sediment, both suspended load and bed load, are collected, and estimates are made of how much sediment is moved by a river. It is also important to look at recent alluvial sediments not now directly associated with the river system. That may be done by digging trenches or collecting samples from floodplains or other alluvial sediments. This type of study looks at the last few hundred or few thousand years of river activity and is critically important to the understanding of these systems. The geologist must study not only what is happening today but also how the system has evolved. In this way, knowledge becomes a predictive tool that greatly enhances the overall understanding of the phenomenon.

It was not until the 1960’s that geologists began to realize how large has been the contribution of alluvial systems to sedimentary rocks. This realization came about as a direct result of studies of modern alluvial systems that, in turn, allowed the correct identification of ancient alluvial deposits. The study of ancient alluvial deposits takes many forms and provides some information that cannot be gathered from modern deposits. For example, modern deposits, for the most part, have only their uppermost surface exposed, except along the bank and in erosion gullies. Ancient accumulations, in contrast, have been transformed into rock such as shale, sandstone, and conglomerate and commonly are parts of uplifted and dissected terrains, including mountain ranges. These exposures of alluvial deposits allow the three-dimensional architecture of alluvial systems to be studied. Geologists look carefully at vertical changes and associations in the types and abundances of sediment that form as a result of alluvial processes.

It has been found that both meandering and braided rivers commonly form cycles of sedimentation that begin with relatively coarse-grained debris and progress to fine-grained debris. These cycles result mainly from the shifting and migration of the channel system. The coarser-grained base represents the channel and bar deposits, and the finer-grained top represents the overbank floodplain deposits.

The study of ancient alluvial deposits also provides clues to the geologic evolution of a region. The specific mineral composition of sedimentary rocks can indicate the nature of the terrain from which the sediment was derived. This, in turn, contributes to an understanding of the history that led to the generation of the ancient river system that produced the alluvial deposit.

Significance

Alluvial systems operate over much of the earth’s surface. They move and deposit enormous quantities of sediment each year and are both a boon and a hindrance to humankind. River valleys and floodplains are desirable regions for agricultural development because of the fertile soil typically found there. Rivers, however, naturally flood about every 1.5 years. This flooding often causes huge losses in property, crops, and sometimes even lives. Flooding associated with alluvial fans can be highly energetic and can occur almost without warning, usually in response to heavy precipitation over a short period. Water levels can build very quickly in narrow valleys with little vegetation and produce a rushing wall of water.

Many different economically valuable materials are found in alluvial deposits. Because alluvial deposits are relatively coarse-grained, they have spaces between the grains that may contain usable fluids, such as water, oil, and natural gas. Many important aquifers are found in alluvial deposits. Petroleum typically originates in marine deposits, but it commonly migrates and may form reservoirs in alluvial deposits. Most sandy alluvial deposits are composed predominantly of quartz grains; however, concentrations of a number of different minerals and ores, including gold and diamonds, occur in alluvial deposits. Another important economic resource in alluvial deposits is the sand and gravel itself. This material is used for road construction and in the manufacture of cement and concrete.

The deposition of sediment from river systems represents the wearing away of the land. Much alluvial sediment eventually makes its way to the ocean, where it undergoes reworking by marine processes and deposition on continental shelves or perhaps in the deep sea. Through time, as continents move relative to one another, these sediments may be subducted into the mantle layer below a continental mass, or become compressed, folded, and uplifted in the formation of major mountain chains. The Appalachian Mountains in the eastern United States and the Himalaya in northern India and China are only two examples of mountains composed in part of sedimentary rocks, some of which are alluvial in origin.

Principal Terms

braided river: a relatively shallow river with many intertwined channels; its sediment is moved primarily as riverbed material

ephemeral stream: a river or stream that flows briefly in response to nearby rainfall; such streams are common in arid and semiarid regions

floodplain: the relatively flat valley floor on either side of a river that may be partly or wholly occupied by water during a flood

longitudinal bar: a midchannel accumulation of sand and gravel with its long axis oriented roughly parallel to the river flow

meandering river: a river confined essentially to a single channel that transports much of its sediment load as fine-grained material in suspension

oxbow lake: a lake formed from an abandoned meander bend when a river cuts through the meander at its narrowest point during a flood

point bar: an accumulation of sand and gravel that develops on the inside of a meander bend

saltation: the movement of non-suspended sediment particles in the direction of fluid flow as particles impact against other particles

transverse bar: a flat-topped body of sand or gravel oriented transverse to the river flow

Bibliography

Charlton, Ro. Fundamentals of Fluvial Geomorphology. Abingdon: Routledge, 2007. Examines the manner in which river systems respond to environmental change and discusses the importance of this understanding for successful river management. Provides a comprehensive overview of river channel management methodologies.

Christopherson, Robert W., and Mary-Louise Byrne. Geosystems: An Introduction to Physical Geography. Toronto: Pearson Education Canada, 2006. Suitable for senior high school and college-level students as well as general readers. Provides a comprehensive overview of alluvial systems in the context of physical geography, with numerous references to online resources throughout.

Darby, Stephen, and David Sear. River Restoration: Managing the Uncertainty in Restoring Physical Habitat. Hoboken, NJ: John Wiley & Sons, Ltd., 2008. Theoretical and philosophical issues with habitat restoration begin this publication and provide a strong foundation for decision making. Later chapters address logistics, planning, mathematical modeling, and construction stages of restoration. Post-construction monitoring and long-term evaluations round out this publication to provide a full picture of the habitat restoration process. This text is highly useful for anyone involved in the planning and implementing of habitat restoration.

Davis, Richard A., Jr. Depositional Systems: A Genetic Approach to Sedimentary Geology. 2d ed. Englewood Cliffs, N.J.: Prentice-Hall, 1992. Offers good introductory chapters on alluvial systems and on related subjects, such as sediment transport. Written for the college-level reader.

Foresman, Timothy, and Alan H. Strahler. Visualizing Physical Geography. New York: John Wiley & Sons, 2012. Uses a unique approach to presenting the concepts of physical geography by heavily integrating visuals from National Geographic and other sources with the narrative to vividly illustrate the manner in which physical geographic processes are interconnected.

Harvey, Adrian M., Anne E. Mather, and Martin R. Stokes. Alluvial Fans: Geomorphology, Sedimentology, Dynamics. Geological Society Special Publication 251. London: Geological Society Publishing House, 2005. Collects papers by experts and researchers in the field. Focuses on resolving how large-scale geological controls such as tectonics and environmental change are reflected in the dynamics of alluvial fan formation. A wide range of rivers are examined. Characteristics of alluvial fans, processes affecting the alluvial fans, and the resulting ecological impacts are discussed.

Pavlopoulos, Kosmas, and Niki Evelpidou. Mapping Geomorphological Environments. New York: Springer, 2009. This text contains a chapter discussing fluvial environments and processes. Glacial formations and the waterfalls of glacial state park are also discussed. Many other geologic formations are described and mapping methodologies are examined.

Reading, H. G., ed. Sedimentary Environments: Processes, Facies, and Stratigraphy. 3d ed. Cambridge, Mass.: Blackwell Science, 1996. Probably the most comprehensive text available on sedimentary environments. Much of the material is technical, but the text has excellent figures and photographs and will not overwhelm the careful reader.

Savenije, Hubert H.G. Salinity and Tides in Alluvial Estuaries. San Diego: Elsevier, 2005. This book is provides clear descriptions of alluvial fan processes. Tidal movements, deposition dynamics, and estuary salinity are discussed. The text has a number of equations, without much explanation of the variables, so a background in hydraulics and water dynamics is needed. This book is well suited for researchers and students looking for examples of the applications of physics in alluvial systems.

Schumm, Stanley A. Active Tectonics and Alluvial Rivers. New York: Cambridge University Press, 2002. Suitable for college-level readers, this source is packed with information on river systems and tectonics.

Tarbuck, Edward J., and Frederick K. Lutgens. Earth: An Introduction to Physical Geology. 10th ed. Upper Saddle River, NJ: Prentice Hall, 2010. An introductory textbook suitable for high school students. Contains sections on channel deposits (bars), floodplain deposits, alluvial fans, and various types of river. Includes many diagrams and photographs. Review questions and a list of key terms conclude the chapter.

Walker, R. G., ed. Facies Models: Response to Sea level Change. 2d ed. Tulsa, Okla.: Society of Economic Paleontologists and Mineralogists, 1994. An excellent compilation on sedimentary systems. Several chapters are devoted to alluvial systems. Suitable for college students.