General aspects regarding "Fluvial Landforms"

(after "Global Geomorphology", Michael A. Summerfield, Longman Publisher 1991)

The fluvial system

Rivers and the landforms they create can be considered at enormous range of scales. In microscale we can examine the movement of individual particles on a riverbed and the relationship between such factors as the roughness of the channel floor, the velocity of water flow and the rate of sediment transport. Generally we can say that fluvial processes create fluvial landforms. These range from the processes operating in a single bend in a river, to the different channel patterns arising from contrasting conditions of waterflow, sediment transport and channel gradient, and ultimately to the morphology of entire drainage basins.

Rivers and streams can be simply defined as bodies of water flowing in an open channel. They have three important roles in landscape creation:

·        They erode the channels in which they flow

·        They transport sediments and solutes provided by weathering and slope processes as well as by the other denutational agents of ice or wind

·        They produce a wide range of erosional and depositional landforms.

River systems are the primary agents of erosion, transportation and deposition in the most landscapes, including many where surface water is not present for most of the time. Over much of the Earth fluvial processes determine the overall form of the land surface.

Fluvial systems can for convenience be regarded as consisting of tree main elements:

·        A zone of sediment production

·        A zone of sediment transfer

·        A zone of sediment deposition

This categorization is, of course, oversimplified because some erosion, transport and deposition occurs in all three zones: nevertheless, within each zone one of these three processes is usually dominant. In large basins the upstream zone in which sediment production predominates is usually a mountainous or upland region. The zone dominated by deposition is generally located along a coast and takes the form of a delta or lowland coastal plain. The depositional zone may, however,, be located at the centre of an interior basin, and local sites of significant deposition may occur in the piedmont environment where rivers emerge at a mountain front. In the intervening zone in which transport predominates, inputs and outputs of sediment may be roughly in balance, at least over the medium term. Sediment may, however, be stored in this zone for long periods of time before being removed downstream into the zone of final deposition.

The catchment area

A catchment area is an area within which water supplied by precipitation is transferred to the ocean, a focus of internal drainage, such as a lake, or to a larger stream. (Note that the term catchment area is synonymous with drainage basin and, in the USA, with watershed)

For a number of reasons the catchment area is the fundamental unit of fluvial geomorphology. Catchment areas are usually well-defined areas, clearly seperated from each other by drainage divides, with which surface or nearsurface flows of water and associated movements of sediment and solutes are contained. Since it is the transfer of material that causes changes in the elevation and the form of the landsurface over time, catchment areas constitute the natural unit for the analysis of fluvially-eroded landscapes. The outlet of a catchment areas provides a very convenient point at which to monitor these movements of water, sediment and solutes. There are important exceptions to these generalizations – such as the partially subsurface flow of water in limestone terrains which may be unrelated to surface topography – but they are valid for the great majority of landscapes. Another important property of catchment areas is their hierarchical nature: each tributary in a catchment system has its own basin area contributing runoff, and so larger basins consist of hierarchy of smaller ones.

A final feature of catchment areas which makes them such important units of analysis in geomorphology is that, following the pioneer work of R.E. HORTON and A.N. STRAHLER, many of their important properties can be expressed quantitatively in a way which allows one basin to be compared with another. Such quantitative description is termed catchment area morphometry and can be applied to the areal and relief properties of basins as well as the characteristics of their  river channel systems.

Channel network characteristics

River systems are a type of network: that is, they consist of a series of links which connect nodes. Networks can be analyzed with respect to two main sets of properties: the topological aspects of stream networks concern the interconnections of the system,  whereas the geometrical aspects involve length, area, shape relief and orientation parameters.

The basic element of stream networks is the stream segment, or link. This is a section of stream channel between two channel junction  or, for “fingertip” tributaries, between a junction and the upstream termination of a channel. Stream order expresses the hierarchical relationship between segments. It is a fundamental property of stream networks since it is related to the relative discharge of a channel segment.

Various systems of streams of stream ordering have been proposed, but the two most frequently used are those of A.N. STRAHLER and R.L.SHREVE. In the STRAHLER  system a stream segment with no tributaries is designated a first order segment. A second order segment is formed by the joining of two first order segments, a third order segment by the joining of two second order segments and so on. It is important to note that with the STRAHLER ordering method there is no increase in order when a segment of one order is joined by another of a lower order. In contrast the stream ordering system proposed by SHREVE defines the magnitude of a channel segment as the number of fingertip tributaries that feed it. As a stream magnitude is closely related to the proportion of the total basin area contributing runoff, it provides a good estimate of relative stream dicharge for small river river systems.

Stream order as defined by STRAHLER has been applied to numerous river systems and has been shown to be statistically related to various elements of catchment areas morphometry. A widely used topological property of stream networks is the bifurcation ratio. This is the ratio between the number of stream segments of one order and the number of the next highest order. This number varies slightly between different successive orders in a basin (that is, between first and second order, second and third order and so on), so a mean bifurcation ratio for a whole basin is normally used. Where Lithology is relatively homogeneous the bifurcation ratio is rarely more than 5 or lee than 3. Nevertheless, a value of 10 or more can be attained in highly elongated basins which can develop where there are arrow, alternating outcrops of soft and resistant lithologies.

Various geometrical properties of stream networks are defined. The most important of these is drainage density. This reflects a balance between erosive forces and the resistance of the ground surface, and, as a consequence, is closely related to climate and lithology. Drainage densities range from lee than 5 km km^-2

On permeable sandstone, to extreme values of more than 500 km km^-2 on unvegetated clay “badlands”. The role of climate is indicated by the very high drainage densities in some semi-arid environments which appear to result from the prevalence of surface runoff and the relative ease with which new channels are indicated. Drainage density is related to the length of overland flow, the latter being approximately equal to the reciprocal of twice the drainage density.

Morphometric properties of catchment areas:

Areal and relief characteristics

Areal properties express the overall plan form and dimensions of catchment areas, while relief properties express elevation differences. The elongation ratio has important hydrological consequences because, in contrast to more circular catchments, precipitation delivered during a storm in highly elongated basins has to travel a wide range of distances to reach the basin outlet. The resulting delay in the arrival of a proportion of the storm flow consequently leads to a flattening of the storm hydrograph.

Relief is related to the slope and stream gradients in a basin, and so indirectly has an influence on the rates of slope processes and sediment transport by rivers. There is also a close relationship between drainage density, mean slope angle and relief. If drainage density is constant and stream channels maintain a constant spacing through time, an increase in local relief due to stream incision must, of necessity, cause an increase in mean slope angles in the basin. There is a limit to this effect, however, as a progressive increase in slope angles cannot continue indefinitely. At a certain point rates of erosion will be so high that influves will be lowered as rapidly as stream channels and local relief will attain a constant value.

Alluvial channels

In alluvial channels the bed and banks are composed of sediment being transported by the river. They can undergo dramatic changes in form as weakly resistant alluvium is eroded, transported and redeposited in redeposited in response to changes in water discharge and sediment load, among other factors. Semi-controlled channels are of intermediate type, being only locally controlled by bedrock or resistant alluvium. Semi-controlled channel will be stable where it is cut into bedrock or resistant alluvim, but over the time it may migrate laterally into alluvium and be much more responsive to changes in hydrological and sedimentological variables.

Alluvial channels: plan form

Alluvial channel exhibit a great variety of plan form. Numerous channel patterns can be recognized but they all represent variations of just a few basic types. One key property is sinuosity which represents the irregularity of channel course and is expressed as the ratio of channel length (measured along the centre of the channel) to valley length (measured along the valley axis). The ratio of valley gradient to channel provides an alternative definition. Sinuosity ranges from 1.0 for perfectly straight channels to around 3  for highly tortuous river courses. Channels with a sinuosity greater than 1.5 are usually described as meandering. Even straight channels generally have a sinuous thalweg, that is, a line of maximum depth along the channel floor.

A second fundamental form that channels may assume involves braiding. This represents the extent to which flow in the channel is divided by islands or bars, that is exposed accumulations of sediment. Islands are vegetated and are relatively long-lived features, whereas bars are less stable, being composed of unvegetated sands or gravels. The degree of braiding is expressed quantitatively as the percentage of channel length that contains islands or bars. If a channel contains islands whose width is more than three times the width of the water at mean discharge it is described as anabranching. The degree of anabranching is expressed as the percentage of channel length occupied by such islands.