Drainage System And Drainage Pattern

Drainage system refers to the origin and development of streams, rivers and lakes through time, while Drainage pattern means spatial arrangement and form of drainage system in terms of geometrical shapes in the areas of different rock types, geological structures, climatic conditions and denudation history.

Drainage Basin

The entire area that provides overland flow, stream flow and groundwater flow to a particular stream is identified as the Drainage Basin or watershed of that stream. The basin consists of the streams, valley bottom, valley sides and interfluves that drain towards the valley.

The drainage basin terminates at a drainage divide which is the line of separation between runoff that flows in the direction of one drainage basin and runoff that goes towards the adjoining basin.

The drainage basin of the principal river will comprise smaller drainage basins with all its tributary streams, and therefore the larger basins include a hierarchy of smaller tributary basins.

Drainage System

W.M. Davis is credited with giving the genetic classification of streams and their valleys. In 1875, Powell introduced the concept of consequent valleys that Davis later expanded it.

Consequent Stream

The streams which follow the course determined by the initial slope of the land are considered the consequent streams. This follows that the streams that develop over newly created land surfaces such as glacial plains, newly emerged sea floors, fresh basalt plateaus, or alluvial plains are consequent streams. It can safely be stated that initially, all runoff is consequent (Bloom, 1978).

Subsequent Stream

As the initial consequent flows down the newly emerged landscape, it erodes the rocks setting in structural control that further defines the flow of the stream. The stream may cut across the easily erodible structure before encountering the next resistant structure.

Subsequent streams refer to those that have shifted from original consequent ones to more readily erodible rocks. They develop independent of and subsequent to consequent drainage patterns.

The drainage patterns dendritic, parallel and radial are considered consequent, while the rest of the patterns described above are associated with structural control and are therefore subsequent streams.

Insequent Stream

According to Davis, the streams that show no apparent structural or lithological control are identified as Insequent streams.

Obsequent and Resequent Stream

The stream that flows in the opposite direction to the original consequent is named an Obsequent stream. In contrast, the stream that flows in the same direction as the original consequent but at lower topographic strata is named a Resequent stream.

Antecedent and Superimposed Drainage

Sometimes the upliftment of land is so slow that the stream is able to maintain its established course by downward erosion, carving deep gorges through the mountains. As these stream antecedents or predates the uplifted landscape, it is known as antecedent drainage.

In other words, the valleys that were eroded may develop new tectonic ridges on their landscape. The river, in some cases, is able to maintain its valley across the tectonic ridge, thereby forming the antecedent stream – that means the river is antecedent to the deformation of the landscape (Bloom, 1978).

Streams that originally existed on a higher landscape that is completely or largely eroded away in such a way that the original drainage pattern gets incised/embedded into underlying rocks of entirely different structure. This drainage system is termed superimposed or superposed and bears no relation to the present local structure (McKnight, 1998).

Drainage Pattern and Structural Relationship

In a particular drainage basin, the streams may flow in a specific arrangement which is termed a drainage pattern. This streamflow over or through the landscape to carve out its valley is predominantly controlled by the geological and topographical structure of the underlying rocks.

As the stream tries to reach the base level, which is generally the sea level, it will encounter several structural obstacles and, in its course of descent, tries to seek the path of least resistance.

In this way, it can be said that most streams are guided by nature and the arrangement of bedrock as they respond directly to structural control. The drainage pattern also reflects the original slope of the land, original structure, and diastrophism, along with the geologic and geomorphic history of the drainage basin.

Two distinct types of drainage patterns identified by geomorphologists are Accordant and Discordant drainage patterns. While the Accordant pattern correlates to the structure and relief of the landscape over which it flows and includes the empirical classification as given by Howard, the Discordant drainage patterns do not correlate with the geology and topography of the region. The genetic classification of antecedent and superposed drainage patterns is included in Accordant.

Empirical Classification of Drainage Pattern

In 1967, Arthur D Howard gave empirical classification and categorising drainage patterns as basic patterns, modifications to basic patterns and varieties of modified patterns. The basic drainage patterns are Dendritic, Parallel, Trellis, Rectangular, Radial, Annular, Multibasinal, and Contorted.

Dendritic Pattern

Tree-like branching or veins of leaf pattern formed by streams is a dendritic pattern. The tributaries randomly join the large stream but always at acute angles – that is, angles less than 90º. This is the most common pattern in the major streams of the world.

The underlying structure does not control the flow pattern of the stream, as rocks are equally resistant to erosion. Uniformly resistant crystalline rocks, with horizontal sediments with gentle regional slope, marks the landscape.

The development of the dendritic pattern is a two staged process. At the outset, the streams flow downwards and increase in numbers. In the final stage, the stream capture of small tributaries by the large ones completes the pattern. The modified basic form of dendritic is Subdendritic, Pinnate, Anastomotic and Dichotomic (Distributary).

The dendritic pattern is observed in most of the Himalayan river systems, where several small tributary streams join the trunk stream. River Bhagirathi flowing through Uttarakhand is joined by several small streams portraying this pattern. Dhansiri basin portrays a dendritic pattern in the upper hilly area where tertiary sediments and low dipping shales are common.

Parallel Pattern

The pattern of streams running parallel to each other is found in areas of moderate to steep slopes with some relief. The streams flow swiftly and straightly with few tributaries, following the same direction. There are intermediate slopes and structural control exerted by subparallel landforms. It is found in areas between linear loess and sand ridges. The modified basic pattern is Subparallel and Collinear.

Trellis Pattern

Most commonly observed in landscapes where the underlying structure comprises alternating bands of tilted hard and soft strata. In areas with dipping or folded sedimentary, volcanic or low-grade meta-sedimentary rocks; in areas of exposed lake or seafloors ribbed by beach ridges. The streams run parallel, linked by short, right-angled segments. The small tributaries on both sides of long parallel subsequent streams are of the same size.

The modified basic pattern of the trellis is Subtrellis, Directional Trellis, Recurved trellis, Fault trellis and Joint trellis.

In the Appalachian mountains, alternating weak and strong strata have been truncated by stream erosion displaying a trellis pattern. Around the Colorado front range in which, strong and weak rocks resting against a core of crystalline rocks portray a trellis pattern.

In Nagaland, the trellis pattern is well developed in the Dayang subbasin, where faulted Disang group of rocks are present. The Nihang Langso subbasin also portrays a distinct directional trellis pattern.

Radial Pattern

The pattern emerges when the stream descends from some sort of concentric uplift, like an isolated volcano, domes or erosion residuals. Mount Egmont on the North Island of New Zealand along the Tasman Sea depicts an extraordinary radial drainage pattern (Macknight, 1998).

The complex of radial patterns in the volcanic fields is referred to as multiracial. Streams on Craters, calderas and other depressions may exhibit this pattern.

The modified basic pattern is centripetal. This is the opposite of Radial, wherein numerous streams converge in a basin. Sometimes occurs at a large scale, such as in the Gulf of Carpentaria basin, which is inundated by the sea in NW Australia and receives streams from hundreds of kilometres.

Annular Pattern

This pattern develops on a dome or in a basin where denudation has exposed alternating concentric bands of tilted hard and soft rocks. Principal streams follow the curving course on soft structure, occasionally breaking through harder layers in short right-angled segments. The longer tributaries to annular subsequent streams indicate the direction of the dip and permit distinction between the dome and basin.

Maverick Spring Dome in Wyoming is a dome of ancient crystalline rock that has been pushed up through a sedimentary overlay. After being deeply eroded, it exposes crystalline in higher parts of hills with upturned concentric sedimentary ridges (called hogbacks) around margins. The streams are mostly incised into softer rocks.

Another example of an Annular pattern is seen in the Race Track or Red Valley, which has ring-like plans that nearly encircle the Black Hills (Thornbury, 1997).

Rectangular Pattern

The rectangular drainage pattern develops on rocks uniformly resistant to erosion. The joints and faults lie at right angles. Since these areas are less resistant to erosion, the stream develops along the joints and may consist of straight segments with right-angle bends with tributaries joining at right angles.

The modified basic pattern is Angulate, and a compound rectangular angulate pattern is quite common.

Norwegian coast and in portions Adirondack mountains, a well-developed rectangular pattern is observed.

Multibasinal Pattern

This multibasinal pattern is suggested for all multi-depression patterns when the exact origin is not clear. It may be on hummocky surficial deposits, deferentially scoured or deflated bedrock, areas of with recent volcanism limestone solution or permafrost. The modified basic patterns, such as glacially disturbed, karst, thermokarst and elongated bay, have been identified.

Contorted

This pattern is irregular and seen in areas with contorted coarsely layered metamorphic rocks. In some areas dikes and veins provide the resistant layer. The longer tributaries to curved subsequent streams indicate a dip of metamorphic layers. The pattern differs from the recurved trellis as it lacks regional orderliness.

Drainage System and Drainage Pattern

Drainage Basin