Fluvial geomorphology emphasizes the dynamic interplay between rivers and landscapes in the shaping of river channels and drainage networks. It includes study of the linkages among channel, floodplain, network, and catchment using a diversity of approaches including stratigraphic analyses, experimental studies of sediment transport in flumes, modeling of physical processes, comparisons of landforms, and sophisticated statistical approaches to gain greater understanding of the physical dynamics of river systems. It helps make sense of the enormous variety exhibited among fluvial systems, and thus the habitat and environmental conditions experienced by the biota. Quantification of the relationships among river features and analysis of the underlying processes contribute to a deeper understanding of how rivers respond to human-induced changes in water and sediment supply that can cause rivers to change their shapes.
A central theme in fluvial geomorphology is that the development of stream channels and entire drainage networks, and the existence of various regular patterns in the shape of channels, indicate that rivers are in dynamic equilibrium between erosion and deposition, and governed by common hydraulic processes. Channel width and depth, velocity, grain size of sediment load, bed roughness, and the degree of sinuosity and braiding are other variables that interact as the river adjusts to the variables that it cannot control, including discharge, sediment load, and its elevational extent.
The drainage basin encompasses a network of channels that join with others downstream in a progression of increasing drainage area and stream size. Stream order is a convenient shorthand for stream size, in which the smallest perennial stream is first order, and the union of two stream of order n forms a stream of order n + 1. Rivers increase in size as one proceeds downstream because tributaries and ground-water add to the flow. Hydraulic geometry equations describe the relationships of width, depth, and velocity to an increase in discharge, either downstream or as flow varies over time at a station. In the downstream case, width increases more than depth, while velocity increases least.
Many features of river channels are familiar to most of us, including sinuosity or meandering, an alternation of riffles, pools, and runs, and the presence of a floodplain provided the river valley is not so V-shaped that it prevents a flood-plain from being formed. These features are formed by the river through cycles of erosion and deposition that in turn are strongly influenced by the balance between the twin supplies of water and sediments. A river's sediment load is the amount of sediment passing a point over some time interval, and includes very fine material that is likely always in transport, and coarser material from the bed and banks that is transported either as suspended load or as bed load, depending on particle size and discharge. The quantity of transported sediment increases with velocity and discharge, but flow events of intermediate frequency actually move more sediment over the years, because extreme events are so infrequent. The dominant or effective discharge is that at which sediment transport is greatest, and it often is approximately the bankfull flood. Human activities can increase or reduce sediment yields. Due to erosion brought about by changing land use, sediment flux into global rivers has increased, while sediment yields to the world's coasts have declined due to the trapping of sediments in impoundments. Some consequences include coastal retreat, subsidence of river deltas, and loss of coastal wetland habitat.
Stream power, the product of discharge and slope, describes the ability of the stream to mobilize and transport material. Sediment transport is directly related to stream power and inversely related to median grain size, and this is a useful relationship for understanding how a stream might respond to changes in sediment and water supply along its length, or due to human interference. As one proceeds from upland, to upland valley, and to large river, the river changes from exporting to accumulating sediments, and from being coupled to hillslopes for its sediment supply to being largely uncoupled. The series of interrelated changes in streamflow and sediment character that occur systematically along the river's length result in a predictable progression of channel types from cascade to step-pool to planform channels, and then to pool-riffle and dune-ripple types. But this classification, like many others, imposes discontinuities on what in reality is continuous change that is still imperfectly understood. River classification as a research topic continues to attract interest because it undeniably can be very useful for management and restoration, and it is a test of our understanding of the processes that are responsible for the great variety of river types.
Finally, to ecologists, geomorphology provides insight into the channel features, habitat units, surface and subsurface zones, floodplains, and riparian corridors that form a complex, shifting mosaic and within which the diversity of the physical template provides the setting in which biologically diverse communities flourish. When this complexity is reduced by dams, channelization, and regulation of river flow, the subsequent homogenization of habitat causes declines in taxon richness, and reminds us of the importance of management actions that view the entire river basin as an integrated unit.
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