Drainage patterns

Detail

What a drainage pattern is

• A drainage pattern is the shape that a river and all its tributaries make on a map. (A tributary is a smaller stream that joins a bigger river.)
• Three things decide the shape:
• Slope: the direction and steepness of the land.
• Rock structure: whether the rock is the same everywhere, or has folds, faults, joints, hard bands and soft bands.

• History: how old the land surface is and what has happened to it, such as uplift, folding or glaciation.

• A river slowly adjusts to the rock under it. It cuts its valley along weak rock and weak lines, such as faults and joints. So the pattern shows the hidden geology [2].

• NCERT names four patterns: dendritic, radial, trellis and centripetal. Geomorphology texts add rectangular, annular, parallel and deranged [2].

1. Dendritic drainage pattern

• Definition: a tree-like pattern. The main river is the trunk and the tributaries are the branches. The word comes from the Greek dendron, meaning "tree".
• Shape: tributaries join the main river at acute angles (angles narrower than a right angle, i.e. less than 90°).
• Why it forms:
• The rock or sediment is uniform, meaning it is about the same everywhere.
• It has no big weak lines, such as faults or joints, to steer the streams [4].

• The slope is gentle, so streams branch freely in every direction.

• Status: it is the most common stream pattern in the world. It looks like the branches of a tree or the veins of a leaf [4].

• India: the rivers of the northern plain (the Ganga–Yamuna system on the Indo-Gangetic plain). The plain is made of thick, even layers of alluvium (fine sand, silt and clay laid down by rivers).

2. Radial drainage pattern

• Definition: rivers start on a high central point, such as a hill, dome or plateau top, and flow outward in all directions, like the spokes of a wheel.
• Why it forms: the land slopes down on every side of the high centre, so water runs off in every direction.
• World setting: radial drainage is typical of volcanic cones, as long as the cone has not been worn down too much [2].
• India's classic example: the Amarkantak plateau
• Location: in the Maikal (Maikala) range, on the Madhya Pradesh–Chhattisgarh border.
• Height: the Narmada rises in the Maikala range near Amarkantak, in Anupur district, Madhya Pradesh, at about 1,057 m [5].
• Rivers flowing out in different directions:

Direction	River	Where it goes
West	Narmada	Arabian Sea. It is the largest west-flowing river of Peninsular India [5]
North	Son	Joins the Ganga as a right-bank tributary
East and south	Mahanadi headwaters, e.g. the Arpa	Mahanadi system, then the Bay of Bengal

• Narmada basin edges: the Vindhyas to the north, the Maikala range to the east, the Satpuras to the south and the Arabian Sea to the west [5].
• Biosphere reserve: the plateau lies in the Achanakmar–Amarkantak Biosphere Reserve, which is part of UNESCO's Man and the Biosphere (MAB) network. It is one of the least disturbed and most ecologically varied landscapes in Chhattisgarh and Madhya Pradesh [6].

3. Trellis drainage pattern

• Definition: the main (primary) tributaries run parallel to each other. Smaller (secondary) streams join them at right angles (90°). The result looks like a garden trellis, a frame of crossing wooden strips.
• How it forms:
• Rock layers are folded into ridges and valleys. The hard bands and soft bands alternate.
• Rivers cut long valleys along the soft bands, so these streams run parallel to the ridges.

• Short streams run straight down the sides of the hard ridges. They meet the valley rivers at right angles.

• World setting: trellis drainage is an adjustment to tight regional folding in which the folds plunge (the folds tilt downward along their length) [2].

• India: the folded Himalayas (longitudinal valleys between parallel ranges) and the Singhbhum belt in Jharkhand.

4. Centripetal drainage pattern

• Definition: streams flow inward from all sides into a central lake or depression (a low-lying hollow in the land). It is the opposite of radial drainage.
• Why it forms: the land slopes down toward one central low point, so all water collects there.
• World settings: drainage that meets at a single outlet or sink, as in some craters, eroded domes with weak cores, parts of limestone country and enclosed desert depressions [3].
• Indian examples:
• Loktak Lake, Manipur
  • It is a large freshwater lake with swamps, fed by several streams [7].
  • It was named a Ramsar site on 23 March 1990 and covers 26,600 ha [7]. (A Ramsar site is a wetland of international importance under the Ramsar Convention of 1971.)
  • Its special feature is the phumdis: thick, floating mats of weeds covered with soil [7].
  • It has been shrinking. It was put on the Montreux Record in 1993 because of catchment deforestation, water hyacinth and pollution [7]. (The Montreux Record is the Ramsar list of sites whose ecology is under threat.)
• Sambhar Lake basin, Rajasthan
  • It is a large saline (salt) lake fed by four streams, a clear case of inward drainage [8].
  • It was named a Ramsar site on 23 March 1990 and covers 24,000 ha [8].
  • It is an important site for the Dalmatian pelican, the greylag goose and the common crane [8].

5. Other patterns (beyond NCERT)

Pattern	Shape	What controls it	Example
Rectangular	Streams bend sharply at right angles	Sets of faults (cracks along which rock has moved) and joints (cracks without movement) that cross at about 90°, often in ancient crustal blocks [2]	Parts of the Vindhyas, the Chambal area
Annular	Streams form rings around a centre	An eroded structural dome (rock layers pushed up into a dome shape). Streams settle on the weak rings of rock [2]	Eroded domes
Parallel	Many short streams run side by side	A steep, even slope in one direction	Short rivers down the Western Ghats to the Arabian Sea
Deranged	A disordered pattern. Channels are broken up by lakes and swamps	Low land from which continental ice has recently melted away. It forms on uneven till (mixed glacial debris) or on ice-scraped hard rock [2]	Glaciated ground

• Rectangular vs trellis: both have right angles. Rectangular comes from cracks (faults and joints) in the rock. Trellis comes from folds with alternating hard and soft bands.
• Annular vs radial: in radial drainage, rivers flow straight out from a dome. In annular drainage, rivers curve around it in rings.

6. Exam pairs (NCERT Exercise 2(ii)–(iii))

Pair	Key difference
Dendritic vs trellis	Rock uniformity: dendritic forms on uniform rock, with acute-angle joins. Trellis forms on alternating hard and soft folded rock, with right-angle joins
Radial vs centripetal	Direction of flow: radial flows outward from a high centre (Amarkantak). Centripetal flows inward to a low centre (Loktak, Sambhar)

• Topographical-sheet practice in identifying patterns is covered in the topographical-maps topic.

Prelims Hooks

• Dendritic = tributaries join at acute angles on uniform rock. It is typical of the northern plain rivers and is the most common pattern worldwide.
• Trellis = parallel primary tributaries, with secondary streams joining at right angles, on folded alternating hard and soft rock (folded Himalayas, Singhbhum).
• Amarkantak (Maikala range, Anupur district, MP, about 1,057 m) → Narmada flows west, Son flows north, Mahanadi headwaters (e.g. Arpa) flow east and south. This is a radial pattern.
• The Narmada is the largest west-flowing river of Peninsular India. Its basin lies between the Vindhyas (north) and the Satpuras (south).
• Centripetal examples: Loktak (Manipur) and Sambhar (Rajasthan). Both became Ramsar sites on 23 March 1990.
• Loktak has phumdis (floating mats) and has been on the Montreux Record since 1993.
• Trap: right-angle bends from faults and joints = rectangular (Vindhyas, Chambal), not trellis.
• Deranged = glaciated ground. Annular = rings around an eroded dome. Parallel = short Western Ghats rivers.
• Radial drainage is classically found on volcanic cones. Centripetal drainage is found in craters and enclosed desert depressions.

Mains Points

• Patterns record geology (GS-I): dendritic shows uniform alluvium in the northern plain. Trellis shows folded Himalayan and Singhbhum rocks. Rectangular shows the jointed, faulted Vindhyan rocks. So reading the pattern helps in mapping rock structure, faults and mineral belts.
• Radial headwater zones are water towers: Amarkantak feeds the Narmada, the Son and the Mahanadi system. Damage to its forest, inside the Achanakmar–Amarkantak Biosphere Reserve, would affect river flows across several states. This makes a case for protecting the catchment and for managing the basins jointly across states.
• Centripetal basins are fragile (GS-III, environment):
• They have no outlet, so silt, nutrients and pollutants collect in the central lake.
• This harms the lake. Loktak faces catchment deforestation, water hyacinth and pollution, and is on the Montreux Record. Sambhar is saline and is under pressure from salt mining.

• Answer: manage the whole catchment, not only the lake.

• Planning use: the pattern affects how fast floodwater gathers. It also affects where dams, canals and interlinking projects (such as Ken–Betwa) can be placed. Parallel short rivers on the Western Ghats give quick, steep run-off. Dendritic plains rivers give wide, slow floods.