alluvial channels Alluvial channels may be defined as channels formed in sediment transported by flowing water. Many channels are actually formed in a combination of bedrock and alluvium: bedrock outcrops may alternate with thick alluvial fills in a downstream direction; or bedrock may be present beneath a relatively thin alluvial veneer, controlling valley morphology or such channel characteristics as the amplitude of meanders. A segment of an alluvial channel may thus be more specifically defined as a channel with the majority of its length formed in alluvium, and beneath which the depth of alluvium exceeds the depth capable of being mobilized during fairly frequent floods.
Alluvial channels are extremely diverse. The grain size of the alluvium, which may range from clay and silt to boulders greater than 1 m in diameter, interacts with sediment supply, valley gradient, and flow regime to shape the morphology of the alluvial channel. Several classification schemes have been developed for alluvial channel morphology. The classification may focus on channel planform, on bedforms within the channel, on flow characteristics, on sediment load, or on vertical or lateral channel stability, among other characteristics. Recognition of consistent channel patterns facilitates the prediction of river behaviour, as well as river management.
At the smallest spatial and temporal scales, alluvial channels are shaped by the movement of water and sediment within the channel. The geometry and surface roughness of the channel boundaries control the velocity distribution across and along the channel. Channels in which the grain size of the alluvial boundary material is less than about a quarter of the flow depth tend to have predictable velocity gradients, with the highest velocities at the top of the water column and in the centre of the channel. The vertical velocity profile for such channels approximates to a logarithmic curve. Irregularities in the channel planform, such as meandering, may set up strong cross-channel currents; coarser bed alluvium may create low-velocity zones near the channel bed and a steep vertical velocity gradient more like an S-shaped curve than a logarithmic curve.
Turbulence associated with irregular cross-channel or vertical velocity distributions erodes and deposits alluvium from the channel boundaries in a self-enhancing feedback effect that produces bedforms. Bedforms are regular, repetitive variations in the channel bed that, by altering boundary roughness, regulate the expenditure of flow energy. Bedforms common along alluvial channels include step–pool sequences, pool–riffle sequences, the succession of riffles–dunes–plane bed–antidunes, and pebble clusters.
The alluvium that composes bedforms comes both from hill slopes within the drainage basin, and from the channel bed and banks. Bed and bank sediments are frequently eroded, transported, and deposited by the flow, creating a constant exchange of sediment at any one channel cross-section. Alluvial channels tend to scour while the water level is rising during a flood as sediment is entrained from the channel bed by the rapidly increasing flow, and to fill during the falling limb as sediment is again deposited. Equations have been developed to predict accurately the entrainment and transport of sand-sized alluvium as a function of velocity, but such prediction becomes less accurate as grain size increases. For an uneven bed composed of gravel-to boulder-sized particles, the velocity may fluctuate dramatically over small spatial and temporal scales, and the movement of individual particles becomes random. Selective entrainment sometimes occurs; particles that protrude further above the bed and into the flow then move first. At other times, particles of different sizes are mobilized almost synchronously.
The roughness of a channel boundary may also be controlled by biotic factors. Large woody debris (more than 2 m long and 10 cm in diameter) may form individual roughness elements, or may collect in regularly spaced steps that create plunging flow. Beavers create broad step–pool sequences by building dams that temporarily pond the flow. Besides increasing channel-boundary roughness, beaver dams trap sediment and slow the downstream transmission of flood waves. The roots of vegetation growing along the channel banks can substantially increase bank resistance, as well as decreasing the velocity of overbank flows and trapping sediment in transport.
Hydraulic geometry was used by Leopold and Maddock in 1953 to explain systematically the role of the magnitude and temporal distribution of discharge as primary controls on channel geometry. Hydraulic geometry describes the relationships between the dependent variables of channel width, depth, mean flow velocity, slope, and resistance, and the independent controlling variable of discharge. Hydraulic geometry can be used to describe cross-sectional or downstream trends in channel morphology. As a general rule, mean velocity and width: depth ratio both increase downstream along alluvial channels as discharge increases. These trends may be complicated in arid zones or karst terrains, where discharge may decrease downstream as a result of infiltration into the channel bed.
Downstream trends of alluvial channel geometry have also been explained in terms of energy expenditure. In 1960 Leopold and Wolman noted that abrupt discontinuities in the rate of energy expenditure along a channel are less compatible with conditions of balance between discharge and channel geometry than is a more or less continuous or uniform rate of energy loss. Subsequent work indicated that adjustments in the dependent variables in response to changes in discharge tend to be as conservative as possible, and that channel geometry is shaped to minimize total energy expenditure. As a result of this work, alluvial channels are generally considered to be malleable by at least the largest flows, so that channel geometry is shaped by flowing waters to represent the most uniform and efficient expenditure of energy under given conditions of water and sediment discharge.
Hydraulic geometry implies the ability for self-regulation within channels, since a change in discharge will cause a corresponding change in the dependent variables. Hydraulic geometry also implies that channels reach an equilibrium state, in which channel geometry is in balance with the prevailing discharge. In practice, however, different components of a channel may experience different lag times in responding to a change in discharge. In general, bedforms and the channel width: depth ratio are most responsive to change; channel gradient, however, has a longer lag or response time.
It may also be difficult to determine to what level of discharge channel geometry is responding. In 1960, Wolman and Miller proposed that the dominant discharge responsible for transporting the majority of sediment along most alluvial rivers was the ‘bankfull discharge’ that fills a river channel up to the top of its banks without spilling over on to the flood plain; this typically recurs at least once every 5 years. Subsequent research has demonstrated that as hydrological variability or channel-boundary resistance, or both, increase, the less frequent floods may dominate channel morphology.
Uncertainties about the magnitude and frequency of discharge that dominate channel morphology may also complicate palaeohydrological inferences of former flow regimes, for the geometry of relict or abandoned alluvial channels is used to infer past discharge regimes.
Stanley Schumm defined ‘river metamorphosis’ as the complete alteration of alluvial channel form as a result of changes in hydroclimatic regime. Metamorphosis may occur over a period of centuries or of decades. In historical times, channel changes have occurred along many of the world's alluvial channels as a result of human interference with flow regimes.
Alluvial channels can also alter their morphology dramatically in the absence of a pronounced change in external controls. A channel reaching an internal threshold may incise independently of changes in discharge or sediment supply. After studying ephemeral alluvial channels in the western United States, Schumm and Parker described a complex response whereby the channels began to incise when gradual aggradation caused the alluvial valley floor to exceed a threshold slope. As the channel incised headward, the increase in sediment supply caused aggradation and braiding in the downstream channel reaches. As incision ceased in the upper reaches, the decrease in sediment supply to the lower reaches triggered a new episode of channel incision. A channel may proceed through two or three such cycles of incision and aggradation before a new balance is established between channel morphology and control variables.
Ellen E. Wohl
Bibliography
Leopold, L. B. and and Wolman, M. G. (1957) River channel patterns: braided, meandering, and straight. US Geological Survey Professional Paper 282–B, pp. 39–73.
Schumm, S. A. (1985) Patterns of alluvial rivers. Annual Review of Earth and Planetary Sciences, 13, 5–27.
Schumm, S. A. and and Parker, R. S. (1973) Implications of complex response of drainage systems for Quaternary alluvial stratigraphy. Nature, 243, 99–100.
Wolman, M. G. and and Miller, J. P. (1960) Magnitude and frequency of forces in geomorphic processes. Journal of Geology, 68, 54–74.
