Initially, water moves down-slope in a thin film as sheet flow, or overland flow. This is called the surface runoff, which concentrates in rills. The rills may develop further into gullies and stream courses.
The resultant drainage pattern or drainage network is the spatial relationship of all streams within a drainage system. In other words, drainage pattern is an arrangement of channels that is determined by a number of variables, including soils, geology, structure, present climate, paleoclimate, tectonic history and human interferences.
It was once believed that a drainage pattern developed at a constant rate throughout time, but modern studies have shown that, while it evolves very rapidly in the early stages, thereafter it changes very little, having achieved a steady state.
There are the following most common drainage patterns that we come across on the land surface of the earth:
(i) Dendritic Pattern (iv) Annular Pattern (vii) Deranged Pattern
(ii) Trellis Pattern (v) Radial Pattern (viii) Centripetal Pattern
(iii) Barbed Pattern (vi) Rectangular Pattern (ix) Superimposed drainage
(i) Dendritic drainage pattern:
This is the most common pattern. This type of drainage pattern is characterized by irregular branching of tributary streams flowing in many directions and at almost any angles, although usually at less than a right angle. Such a pattern develops upon rocks of uniform resistance and demonstrates lack of structural control.
Dendritic drainage patterns are mostly found upon nearly horizontal sedimentary rocks or in areas of massive igneous rocks. This pattern may also be found on folded or metamorphosed rocks, especially when imposed upon them by superposition.
These patterns resemble the vein patterns in leaves. Remember that the term dendritic has been taken after the Greek word ‘dendron’ meaning a tree. The drainage pattern of in sequent streams being tree-like in form is called dendritic drainage.
(ii) Trellis drainage pattern:
The trellis drainage pattern is characteristic of a dipping or folded topography, which exists in nearly parallel mountains, where drainage patterns are controlled by rock structures of variable resistance.
The main streams are directed by the parallel folded structures, whereas smaller streams are at work on adjacent slopes, joining the main stream at right angles. It is most commonly found in a scarp-and-vale terrain where drainage has become adjusted to structure.
In this drainage the consequent streams follow the direction of dip and the subsequent streams follow the direction of strike, before joining the principal stream at right angles, and the obsequent streams flow in a counter-dip direction.
In the trellis pattern of drainage the rivers form a net-like system and the tributary channels of subsequent and obsequent categories flow roughly parallel to each other. This pattern is found in areas where hard and soft rocks occur in almost parallel bands.
The trellis drainage pattern can be seen in the Appalachian region of the U.S.A., where hard and soft rocks occur in parallel bands.
This pattern is also found in glaciated areas characterized by the presence of drumlins, the parallel hills. The trellis pattern may also be found to some degree in areas of parallel sand dunes provided surface drainage lines exist on such permeable materials.
(iii) Barbed drainage pattern:
The barbed pattern may be found at or near the headwater portions of the drainage systems. The tributaries join the main stream in ‘Boat hook bends’ which point upstream.
Most of barbed drainage pattern is the result of river-capture which reverses the direction of flow. However, the tributary channels continue to flow in their original direction.
(iv) Annular drainage pattern:
Annular pattern represents that part of a drainage pattern in which the subsequent streams follow the curving or arcuate courses before joining the consequent stream. These results from a partial adaptation to an underground circular structure i.e. batholiths.
This drainage pattern is an example of structural control. The subsequent streams find it easier to erode the concentric, less resistant strata. This pattern may be found around maturely dissected domes which have alternating belts of strong and weak rock encircling them. The Black Hills is encircled by annular pattern of drainage system.
(v) Radial drainage pattern:
A radial drainage pattern results from streams flowing off a central peak or dome, such as occurs on a volcanic mountain. The radial drainage pattern resembles the spokes of a wheel. Such a drainage pattern develops on the side of a dome or a volcanic cone.
This pattern may also be found on various other types of isolated conical or sub- conical hills. The Lake District of New England is the typical example, which has been formed by the superimposed drainage. Another fine example of the radial drainage pattern is found in Sri Lanka where the streams rising over the Central Highlands radiate in all directions.
(vi) Rectangular drainage pattern:
Rectangular patterns occur where streams are guided by intersecting joints, usually in plutonic or metamorphic rocks. It is characterized by right- angled bends and right-angled junctions between tributaries and the principal stream.
It results from the structural control imposed by the jointing or fault pattern of the underlying rocks. It differs from trellis pattern drainage, since it is more irregular and its tributary streams are not as long or as parallel as in trellis drainage.
In this drainage both the main stream and its tributaries show right-angled bends. A sub-type of rectangular drainage is the angulated pattern. Rectangular drainage pattern develops where joints and faults join each other at acute or obtuse angles rather than at right angles.
(vii) Deranged drainage pattern:
It represents a drainage pattern which is found in the glaciated shield regions of Canada and Northern Europe with no clear geometry in the drainage and no true river valley pattern. This uncoordinated pattern is characteristic of a region that has been recently vacated by an ice-sheet.
This is so because of the irregularities produced by the glacially deposited materials such as kame-and kettle terrain. It is also due to the fact that no sufficient time was available for the drainage to become adjusted to the structures of the underlying glacial drift.
The picture presented by the drainage system is very confusing, since there are numerous water courses, lakes and swamps, some interconnected, and some in local drainage basins of their own. In fact, in such glaciated regions the pre-glacial drainage has been completely destroyed and the new drainage could not have time to develop any significant degree of integration.
(viii) Centripetal drainage pattern:
This is the drainage pattern, as the term ‘centripetal’ implies, in which the streams drain radically inwards, either towards a single main stream which drains the basin, or to a lake which may or may not have an outlet.
Such patterns are found on sinkholes, craters and other basin-like depressions. This drainage pattern is also called endorheic drainage which refers to an inward flowing pattern of drainage in the world’s semi-arid zones.
Seasonal rivers, subject to flash floods, flow towards larger basins, occupied by playa lakes and thick deposits of alluvium.
(ix) Superimposed drainage:
It is also known as discordant drainage which shows discordance with the underlying rock structures, because this drainage system originally developed on a cover of rocks that has now disappeared owing to denudation.
Consequently, river directions relate to the former cover rocks, as these cover rocks were being eroded, the rivers have been able to retain their courses notwithstanding the newly exposed structures.
The present drainage pattern has been superimposed on the older rocks as an inheritance from the vanished cover. The rivers and lakes of the Lake District in Britain afford the clearest examples of the superimposed drainage.