Chapter 2 — 1.2 Works of a River

A river is a natural stream of water that flows in a definite channel from a higher place to a lower one under gravity and finally empties into a sea, a lake or another river. Running water is the most powerful and widespread of all exogenetic agents, because rain falls almost everywhere and all of it that does not evaporate or sink into the ground must run off along the surface. Rivers have carved most of the valleys of the world and built most of its plains.

A river begins at its source, which may be a spring, a glacier, a lake or simply a hillside where rainwater collects into rills. The Ganga rises from the Gangotri glacier at Gaumukh, the Teesta from the Zemu glacier in Sikkim, the Damodar from the Chotanagpur plateau near Chandwa in Jharkhand. The place where a river ends is its mouth. A smaller stream that joins a river is a tributary; the Teesta is a tributary of the Brahmaputra, and the Rangeet is a tributary of the Teesta. A branch that leaves the main river and flows separately, usually in the delta, is a distributary; the Bhagirathi-Hooghly is a distributary of the Ganga. The point where two rivers meet is a confluence.

The entire area drained by a river and all its tributaries is its river basin or drainage basin. The Ganga basin covers about 8.6 lakh square kilometres in India alone. The highland that separates one basin from the next, so that rain falling on one side drains into one river and on the other side into another, is the watershed or water divide. The Western Ghats form the watershed between the short west-flowing rivers and the long east-flowing rivers of the peninsula. The area from which a river collects its water, including its tributaries, is its catchment area; for a single stream the terms catchment and basin are used interchangeably.

The pattern made by a river and its tributaries on the map is the drainage pattern. The commonest is the dendritic pattern, branching like a tree, which develops on uniform rock; the Ganga system shows it. A trellis pattern of tributaries joining at right angles develops on folded rocks, a radial pattern flows outward from a dome or volcanic cone like the spokes of a wheel, as from the Amarkantak plateau, and a centripetal pattern flows inward into a basin or lake.

A river carries out three kinds of work: it erodes its bed and banks, it transports the eroded material as its load, and it deposits the load where it loses energy. Which of the three dominates at any point depends on the river's velocity, and velocity depends on slope and volume. So the work changes from the mountains to the sea, and to understand it we divide the river into three courses, treated in the next topic.

From source to mouth a river passes through three stretches, each with a different slope, velocity and kind of work. These are the upper course or mountain course, the middle course or plain course and the lower course or deltaic course.

In the upper course the river flows through mountains. The slope is steep, so the velocity is high although the volume of water is small. With this energy the river cuts vertically into its bed, a process called vertical erosion or downcutting. The valley is narrow, deep and V-shaped, and where the rock is very hard it becomes a gorge or canyon. Waterfalls, rapids, potholes and plunge pools are common. The river carries large angular boulders, which are used as tools to deepen the bed. Erosion is the dominant work; deposition is almost absent. The Ganga above Haridwar, the Teesta above Sevoke and the Brahmaputra in Arunachal Pradesh are in their upper course.

In the middle course the river enters the plain at the foot of the mountains. The slope decreases sharply, so velocity falls, but the volume increases as tributaries join. Vertical erosion almost stops and lateral erosion, the cutting of the banks sideways, becomes important. The valley widens, the river begins to swing in curves called meanders, and a floodplain develops on either side. Transportation is the main work, with erosion on the outer banks of bends and deposition on the inner banks. The load is now made of rounded pebbles, sand and silt. The Ganga from Haridwar to Rajmahal and the Damodar below Durgapur are in their middle course.

In the lower course the river approaches the sea. The slope is almost nil, the velocity is very low and the volume is at its maximum. The river can no longer carry its whole load and deposition becomes the dominant work. The channel is broad and shallow, the meanders are very wide, oxbow lakes and natural levees are common, and near the mouth the river splits into distributaries and builds a delta. The load is fine silt and clay. The Ganga below Rajmahal, with the Bhagirathi-Hooghly and the Sundarbans, is in its lower course.

The three courses can be compared at a glance:

The boundaries between the courses are not sharp; a river passes gradually from one to the next, and a large river may show middle-course features for hundreds of kilometres.

River erosion is the wearing away of the bed and banks of the channel and the removal of the loosened material by the flowing water. It takes place in four ways, which usually act together.

Hydraulic action is erosion by the sheer force of moving water. Fast water striking the bed and banks loosens particles, forces itself into cracks and joints, and compresses the air trapped there; when the water withdraws the air expands suddenly and the rock is shattered. This is very effective on jointed rocks and on the soft, unconsolidated banks of rivers in the plains. In the monsoon the swollen Ganga and Padma eat away hundreds of metres of bank every year in Malda and Murshidabad by hydraulic action; the phenomenon is locally called bhangan.

Abrasion or corrasion is erosion by the load. The pebbles, gravel and sand carried by the river act like the grit of a sandpaper, scraping and grinding the bed and banks. Where the current swirls in eddies, the pebbles are spun round and drill circular holes in the bed called potholes. Abrasion is the chief means by which a river deepens its bed in the upper course, and the more load a river carries, the more strongly it abrades.

Attrition is the wearing down of the load itself. As the boulders and pebbles roll and strike against one another, they are broken and rounded, becoming smaller as they travel downstream. This is why the boulders of the upper course become the rounded pebbles of the middle course and the sand and silt of the lower course. Attrition does not erode the bed directly, but it produces the finer material that the river can carry farther.

Solution or corrosion is chemical erosion. River water containing dissolved carbon dioxide and organic acids dissolves soluble rocks such as limestone, dolomite, rock salt and gypsum. The dissolved matter is carried away invisibly as the solution load. Solution is most important in limestone regions like the Meghalaya plateau.

Erosion may be vertical, deepening the valley, which dominates in the upper course where the slope is steep, or lateral, widening the valley by cutting the banks, which dominates in the middle and lower courses where the river swings from side to side. A third, headward erosion, is the lengthening of the valley backwards at the source by springs and rills cutting into the hillside; it may lead one river to cut into the basin of a neighbouring river and capture its headwaters, a process called river capture or river piracy.

The rate of erosion depends on the velocity of the water, the amount and hardness of the load, the nature and structure of the rock, and the volume of water. Doubling the velocity roughly increases the erosive power many times, which is why a river in flood does more erosional work in a few days than in the rest of the year.

The first and most characteristic landform of river erosion is the valley itself. In the upper course the river, flowing with high velocity down a steep slope, concentrates its energy on cutting downward. At the same time weathering, rain-wash and mass wasting attack the two sides of the cut and make them slope back. The result is a valley whose cross-section is shaped like the letter V: a narrow floor occupied almost entirely by the river, and steep sides rising on both hands. This is the V-shaped valley. The more rapid the downcutting compared with the wearing back of the sides, the narrower and steeper the V. Spurs of the hills project alternately from either side into the valley, and the river winds round them; seen from downstream, they appear to overlap and hide the valley, and are called interlocking spurs.

Where the rock is very hard and resistant, or where the climate is dry so that the valley sides are not worn back by rain, or where the land is being uplifted so that the river keeps cutting down, the valley becomes extremely deep and narrow with almost vertical walls. Such a valley is a gorge. The Indus has cut a gorge more than 5,000 m deep near Nanga Parbat, and the Brahmaputra gorge in the eastern Himalaya is one of the deepest in the world. In West Bengal the Teesta flows in a gorge through the Sivalik hills above Sevoke, and the Rangeet has cut a gorge through the Darjeeling hills.

A canyon is a gorge of very large size, formed where a river cuts through horizontally bedded rocks in an arid region. Because the rock layers differ in hardness, the walls of a canyon are stepped rather than smooth, and because there is little rain, the walls are not worn back and remain almost vertical for great depths. The Grand Canyon of the Colorado river in the United States is about 446 km long, up to 29 km wide and more than 1,800 m deep. The term I-shaped valley is sometimes used for a gorge or canyon with vertical walls, in contrast to the V-shaped valley with sloping sides.

The differences between a gorge and a canyon are chiefly of size, climate and rock structure: a canyon is larger, forms in dry climates, has stepped walls due to horizontal strata, and is typically wider at the top; a gorge is smaller, forms in any climate on hard rock, and has more uniform, nearly vertical walls.

The V-shaped valley, gorge and canyon are all signs of a youthful river and of the dominance of vertical over lateral erosion. As the river matures and lateral erosion increases, the V opens out into a wide valley with a flat floor, the stage described in the topics on meanders and floodplains.

A waterfall is the vertical fall of a river from a height where there is a sudden break in the slope of its bed. Waterfalls belong to the upper course and are marks of a youthful river. They form under several conditions.

The commonest cause is the difference in hardness of rocks. When a layer of hard rock lies horizontally over a layer of soft rock across the bed, the river erodes the soft rock faster; the hard rock projects as a ledge over which the water falls. The falling water erodes the soft rock at the base still faster by hydraulic action and by the swirling of pebbles, and cuts a deep basin called a plunge pool. Undercut from below, the hard ledge periodically collapses, and so the waterfall slowly moves upstream, a process called the recession or retreat of the waterfall. Behind it is left a gorge whose length records the total retreat. The Niagara Falls on the border of the United States and Canada, formed on hard limestone over soft shale, have retreated about 11 km since their formation and continue to move back at about one metre a year.

Waterfalls also form where the river flows over the edge of a plateau or an escarpment, as the Jog (Gersoppa) Falls on the Sharavati in Karnataka, 253 m high, at the edge of the Western Ghats, and the Chachai and Dhuandhar falls on the plateau edges of Madhya Pradesh. A fault across the river bed, which lifts one side higher than the other, produces a fall along the fault scarp; the Victoria Falls on the Zambezi lie along a fault line. In glaciated mountains a tributary glacier's valley is left hanging high above the main valley, and after the ice melts the tributary stream falls from the hanging valley as a waterfall, as in the Yosemite Falls of California. In West Bengal, small falls such as those on the streams of the Darjeeling hills near Kurseong and the Bandhaghati falls of Purulia are of the hard-and-soft-rock type.

A cascade is a series of small falls in steps, and a cataract is a very large volume of water falling over a low height, as the cataracts of the Nile. Rapids are stretches of fast, broken, turbulent water where the bed drops steeply but not vertically, usually because bands of hard and soft rock alternate across the channel and the soft bands have been lowered more. Rapids are common in the Teesta above Sevoke, where they are used for river rafting.

Waterfalls are important as sources of hydroelectric power, because the height of fall provides the energy; they also attract tourism. As a river matures, its waterfalls recede, lose height and finally disappear, leaving a gorge.

Several smaller erosional features belong to the upper course and are regularly asked in the examination.

Potholes are circular or cylindrical holes drilled into the rocky bed of a river. Where the current forms an eddy or whirlpool, usually over a slight hollow or a joint in the rock, the pebbles carried by the water are spun round and round in the same spot. Acting like the bit of a drill, they grind the hollow deeper and wider. The pebbles themselves are worn round and smooth and are often found lying at the bottom of the hole. Potholes range from a few centimetres to several metres in diameter and depth, and neighbouring potholes may join to deepen the whole bed. They are common where a river crosses hard rock, as in the beds of the Subarnarekha, Kangsabati and Damodar on the Chotanagpur plateau, and in the Bhedaghat marble rocks of the Narmada. The name comes from their resemblance to cooking pots. A related feature, the plunge pool, is a very large pothole cut at the foot of a waterfall.

Interlocking spurs are the projecting ridges of high ground that extend alternately from the two sides of a V-shaped valley. In the upper course the river is not powerful enough to cut straight through the hard projecting spurs of the hills, so it winds round them. Looking along the valley, the spurs from the left and right appear to interlock like the fingers of two clasped hands. When a glacier later occupies the valley, it cuts the spurs off and leaves truncated spurs, a feature studied in the next section. Interlocking spurs can be seen in most Himalayan valleys, including the upper Teesta and the Rangeet.

River capture, also called river piracy or stream capture, is the process by which a powerful river, extending its valley backward by headward erosion, cuts through the watershed into the basin of a neighbouring weaker river and diverts its headwaters into its own channel. The capturing river is the captor or pirate stream, the captured stream is the misfit or beheaded stream, the sharp bend at the point of capture is the elbow of capture, and the dry valley left below the point of capture is the wind gap. Capture happens when the captor river has a steeper gradient, more rainfall or softer rock, so that it erodes headward faster. In India, the Arun river of Nepal is believed to have captured the drainage of the southern Tibetan plateau, and geologists suggest that the Yamuna was once a tributary of the Indus system before it was captured by the Ganga. On the Chotanagpur plateau small examples of capture among the tributaries of the Damodar and Subarnarekha are recognised.

These features share one lesson: in the upper course, the river's energy goes into cutting down and cutting back, and the shape of the landscape is set by the pattern of hard and soft rock over which it flows.

Everything that a river erodes it must carry, and the material it carries is its load. A river's load has three parts. The dissolved load consists of minerals carried in solution, invisible to the eye; it comes from the chemical weathering of the basin and from solution of soluble rocks. The suspended load consists of fine clay, silt and fine sand held up by the turbulence of the water and carried along with it; it makes the water muddy and is by far the largest part of the load of the Ganga and Brahmaputra. The bed load consists of the coarse sand, gravel, pebbles and boulders that move along the bottom.

The particles are moved in four ways. Dissolved minerals move in solution. Fine particles move in suspension. Sand grains too heavy to remain suspended move by saltation, a series of hops and jumps along the bed; each grain that lands knocks others into the flow. Pebbles and boulders move by traction, rolling, sliding and dragging along the bed, and they move only when the current is strong.

Two terms measure a river's transporting power. Capacity is the total quantity of load a river can carry, and competence is the diameter of the largest particle it can move. Both depend chiefly on velocity, and competence increases enormously with velocity: it is estimated that the size of the largest particle a stream can move varies as the sixth power of the velocity, so that doubling the speed allows it to move a boulder 64 times heavier. This is why a mountain river in monsoon flood rolls boulders the size of cars that lie motionless in the dry season. Capacity also increases with volume; the great rivers of the plains, though slow, carry vast tonnages of fine sediment because their volume is huge.

The total sediment carried by Indian rivers is among the highest in the world. The Ganga–Brahmaputra system delivers about 1,000 to 1,600 million tonnes of sediment a year to the Bay of Bengal, and the Kosi is so laden with Himalayan debris that it constantly shifts its channel across north Bihar. The load of a river changes along its course: angular boulders in the mountains, rounded pebbles at the foothills, sand in the plains and silt and clay in the delta. This sorting by size is the direct result of attrition and of the falling competence of the river as its slope decreases.

Transportation is the link between erosion and deposition. It carries the products of degradation in the highlands to the sites of aggradation in the lowlands; the alluvium of the entire Ganga plain and delta is Himalayan rock that has travelled hundreds of kilometres in the river.

A river deposits its load when its velocity falls and it can no longer carry the material. Velocity falls when the slope decreases, as at the foot of a mountain; when the channel widens and the water spreads out; when the volume decreases in the dry season or by percolation; when the river enters still water such as a lake or the sea; and when an obstruction such as vegetation or a bar slows the flow. Deposition begins with the heaviest particles and ends with the lightest, so river deposits are sorted, with gravel near the mountains, sand in the plains and clay in the delta. The material deposited by a river is called alluvium, and all landforms built of it are alluvial landforms.

The first depositional landform, met at the very point where the river leaves the mountains, is the alluvial fan. When a swift mountain stream emerges from its narrow valley on to the plain, the slope drops suddenly and the channel spreads out from its confined gorge. The river loses much of its velocity at once and drops its coarse load of boulders, gravel and sand. This material accumulates in a fan-shaped or cone-shaped deposit with its apex at the mouth of the valley and its broad base spreading over the plain. The stream splits into several shifting channels across the fan. Where many neighbouring streams build fans that join sideways, a continuous apron of deposits forms along the mountain foot, called a piedmont alluvial plain or, in desert regions, a bajada.

The alluvial fans at the foot of the Himalaya are of enormous size. The Bhabar belt, a narrow strip of coarse gravel and boulders along the foot of the Sivaliks from the Indus to the Teesta, is a chain of coalescing fans; the streams sink into its porous gravel and reappear farther south in the marshy Terai. In north Bengal, the Teesta, Jaldhaka, Torsa, Raidak and Sankosh have built the gravel fans of the Duars in Jalpaiguri and Alipurduar, on which tea gardens are planted. Old Teesta fans can be traced across the whole of the north Bengal plain.

Fans are important for settlement because they are well drained and their groundwater is easily reached; many hill-foot towns of the world stand on them. But they are also hazardous, because the streams that build them shift channels without warning during floods, and debris flows can sweep down the fan surface.

A related feature in a lake is the lacustrine delta, built where a stream enters a lake and deposits its load; over time the lake fills and becomes a flat plain. The Kashmir valley floor is a former lake filled in this way.

In its middle and lower courses a river rarely flows straight. On the gentle slope of the plain the current is easily deflected by any obstacle or by the slightest unevenness of the bed, and once the water begins to swing to one side, the swing grows. The result is a series of sweeping loops or bends called meanders. The name comes from the river Maeander (Büyük Menderes) in Turkey, famous for its winding course.

A meander grows by the unequal action of the current on its two banks. As the water rounds a bend, its momentum throws the strongest current against the outer bank or concave bank, which is undercut and eroded into a steep river cliff. At the same time the current on the inner bank or convex bank is slack, so sand and silt are deposited there as a gently sloping point bar or slip-off slope. Erosion on the outside and deposition on the inside make the bend deeper and wider, and the whole meander also creeps slowly downstream. The belt of the floodplain within which the river swings is the meander belt.

As the meander loops grow, the neck of land between two neighbouring loops becomes narrower and narrower. Finally, during a flood, the river breaks through the neck and takes the new straight and shorter path. The abandoned loop is cut off from the main channel, its two ends silt up, and it remains as a crescent-shaped lake called an oxbow lake, from its likeness to the U-shaped yoke placed on the neck of an ox. In time the oxbow lake fills with silt and vegetation to become a marsh and then a curved depression, a meander scar, on the floodplain. In Bengal such lakes are called beel or baor; the Bhagirathi and Jalangi in Nadia and Murshidabad have left many oxbow lakes, the Jhilli beel and Chhariganga baor being examples, and the floodplain of the Ganga in Malda and Bihar is dotted with them.

Meanders are typical of the mature and old stage of a river and of its middle and lower courses; they need a gentle slope, a large volume of water, easily eroded alluvial banks and a plentiful load of fine sediment. A river whose meanders have been cut down into hard rock because the land was uplifted after the meanders formed is said to have incised or entrenched meanders.

Meandering rivers shift their channels continually, which makes them dangerous for bank-side villages, as at Malda and Murshidabad, but the process also spreads fresh silt over the floodplain and keeps it fertile.

In the plains the river builds the surface over which it flows. Three landforms describe this work.

The floodplain is the wide, flat, low-lying plain on either side of a river in its middle and lower courses, built of alluvium and covered by water when the river floods. It forms in two ways. First, as the river meanders, lateral erosion widens the valley into a broad flat floor, and the point bars deposited on the inner banks of successive meanders coat it with sand and silt. Second, whenever the river overflows its banks, the water spreads over the plain, loses velocity and drops a thin layer of fine silt and clay across it. Each flood adds a new layer, and the plain is slowly raised. Because of these yearly additions of fresh silt, floodplains are among the most fertile lands on earth and support the densest agricultural populations; the whole of the Ganga plain and the plains of Bengal are floodplain. The older, higher alluvium not reached by present floods is called bhangar, and the newer, lower alluvium flooded each year is khadar.

Natural levees are low ridges or embankments of sediment that run along both banks of a river in its lower course. When flood water spills out of the channel, the sudden loss of velocity at the bank causes the coarsest part of the load, the sand, to be dropped immediately along the edge, while the finer silt is carried farther on to the plain. Flood after flood, the bank is built up higher than the plain behind it. The river thus comes to flow on a ridge of its own making, sometimes several metres above the surrounding land; the Mississippi and the Hwang Ho are classic examples, and in Bengal the Bhagirathi and the Damodar in its lower course have well-marked levees on which villages and roads are placed. The low ground behind the levee, often waterlogged, is the back swamp. Tributaries may be unable to cross the levee and flow parallel to the main river for a long distance before joining it, forming a yazoo stream. Levees protect the plain from ordinary floods but, when a large flood breaks through them, the water pours into the back swamp and cannot easily return, causing prolonged inundation, a frequent problem in the Damodar and Kosi plains.

A braided channel forms where a river carrying a very large load of sand and gravel, with a variable volume, deposits so much material in its own bed that the channel is split into a network of small, shifting channels separated by sand bars and islands. The islands are called chars in Bengal and are used for cultivation when exposed. The Brahmaputra in Assam, with a channel up to 10 km wide, is the finest example in India; the Teesta in the Jalpaiguri plain and the Ganga near Malda are also braided. Braiding indicates that the river is overloaded, that is, it receives more sediment than it can carry.

When a river reaches the sea or a lake, its velocity drops almost to zero and it deposits its entire remaining load at the mouth. If the sea does not remove this material as fast as it arrives, the deposits grow into a low, flat, triangular plain across which the river divides into many branches. This landform is the delta. The name was given by the Greek historian Herodotus, who saw that the mouth of the Nile resembled the Greek capital letter delta (Δ). The branches into which the river splits are its distributaries.

A delta grows in a definite way. The coarser sand is dropped first, forming a sloping front of thick beds called the foreset beds; the finer clay is carried a little farther out and settles as thin, nearly horizontal bottomset beds; and as the river flows over the deposit it lays down horizontal topset beds on top. The deposit builds outward into the sea, and the river, whose channel is choked by its own sediment, splits repeatedly into distributaries. The salt of sea water makes the fine clay particles clump together and settle faster, a process called flocculation, which speeds up the growth.

Not every river builds a delta. The conditions favourable for delta formation are: (1) the river must carry a large load of sediment, which needs a long course and an easily eroded basin; (2) the velocity must fall sharply at the mouth, so a gentle lower course is needed; (3) the sea at the mouth must be shallow, so that deposits soon reach the surface; (4) the coast must be sheltered, with weak tides, waves and currents that do not carry the sediment away; (5) the river should have a large volume of water and a steady flow; and (6) the river mouth should not be a subsiding basin in which the sediment sinks out of sight. The Ganga–Brahmaputra satisfies all of these, and so it has built the largest delta on earth, covering about 1,00,000 km² in West Bengal and Bangladesh, whose seaward fringe is the Sundarbans. The Mahanadi, Godavari, Krishna and Kaveri also build deltas on the sheltered east coast of India, while the Narmada and Tapi, flowing into the Gulf of Khambhat where tides are strong, form estuaries instead.

Deltas are extremely fertile because they are built of the finest silt, and they support some of the densest populations in the world; the rice lands of southern Bengal are deltaic. They are also low-lying and exposed to floods, cyclones and rising sea level; the Sundarbans are threatened by all three. The delta of the Ganga is active in its eastern part, where the Meghna still adds silt, and moribund or dying in the west, where the Bhagirathi and its old distributaries such as the Jalangi and Mathabhanga no longer receive enough water from the Ganga to carry silt to the sea. The whole delta is still growing seaward at several metres a year in the Meghna estuary.

Deltas are classified by their shape, which depends on the balance between the river's supply of sediment and the sea's ability to spread or remove it.

• An arcuate delta is fan-shaped or bow-shaped, with a smoothly curved seaward margin, and is the commonest type. It forms where the sediment is coarse (sand and silt) and waves and currents are moderate, so that the deposit is spread evenly along the front. The Nile, Ganga, Mahanadi, Godavari, Krishna, Kaveri, Rhine and Hwang Ho deltas are arcuate.
• A bird's-foot delta is formed where the river carries very fine sediment into a calm sea with few waves and currents. The distributaries build narrow levees far out into the sea, and the shape resembles the spread claws of a bird's foot. The Mississippi delta is the classic example.
• A cuspate delta is tooth-shaped or pointed, with a sharp seaward apex, formed where a single channel enters a sea whose waves strike the coast head-on and push the deposits back on both sides. The Tiber delta of Italy and the Ebro delta of Spain are examples.
• An estuarine delta forms inside a drowned river mouth or estuary, filling it slowly from the head; the Seine and Elbe deltas are examples.
• A lacustrine delta forms where a river enters a lake.

An estuary is the funnel-shaped mouth of a river in which the tide enters and mixes sea water with river water. It forms where the river empties into a deep, open sea with strong tides and currents that sweep away the sediment as fast as it arrives, so that no delta can build. Estuaries also form where the coast has subsided or the sea has risen, drowning the lower valley. The Narmada and Tapi form estuaries in the Gulf of Khambhat, the Thames in England and the St. Lawrence in Canada are famous estuaries, and the Hooghly below Kolkata is tidal and estuarine in character, which is why the port of Kolkata depends on dredging and the tide. Estuaries make excellent natural harbours because they are deep and sheltered, and they are rich fishing grounds, whereas deltas are shallow and shifting and poor for ports.

The differences between delta and estuary are frequently asked:

The physical map of West Bengal is almost entirely a record of river work, and the examination often asks the student to name West Bengal examples of the landforms in this section.

The rivers of north Bengal, the Teesta, Jaldhaka, Torsa, Raidak, Sankosh and Mahananda, rise in the Himalaya of Sikkim, Darjeeling and Bhutan. In the mountains they show every upper-course feature: the V-shaped valleys and interlocking spurs of the Rangeet and upper Teesta, the Teesta gorge through the Sivaliks at Sevoke, rapids used for rafting, and small waterfalls on the tributary streams. On leaving the hills they build the great gravel fans of the Duars and the Terai, then flow braided across the north Bengal plain, shifting their channels; the Teesta itself moved from the Ganga system to the Brahmaputra system after the flood of 1787. The Mahananda has cut through the old Barind alluvium of Malda, and the Ganga at Manikchak in Malda and at Farakka shows the most violent bank erosion in the state.

The rivers of the western plateau, the Damodar, Ajay, Mayurakshi, Kangsabati, Dwarakeswar, Silabati and Subarnarekha, rise on the Chotanagpur plateau or its fringe. In Purulia, Bankura and Birbhum they are rocky, with potholes and rapids on hard gneiss and small falls at the plateau edge; on the laterite the monsoon rain has gullied the land into the khoai badlands. Reaching the plains they become mature, meandering rivers with wide floodplains and natural levees; the Damodar, once called the sorrow of Bengal for its floods in Bardhaman, Hooghly and Howrah, has been controlled by the dams of the Damodar Valley Corporation. Their combined alluvium forms the Rarh plain.

The Ganga delta occupies the whole of southern Bengal. Below Farakka the Ganga sends off the Bhagirathi-Hooghly as its main distributary in India, while the Padma continues into Bangladesh. The delta between the two is crossed by old distributaries, the Jalangi, Mathabhanga, Churni and Ichhamati, which now carry little water; this is the moribund delta of Nadia and Murshidabad, full of oxbow lakes called beels and baors. The mature delta of Hooghly, Howrah and North 24 Parganas is fully built and cultivated, with the Hooghly flowing on its levees past Kolkata. The active delta of South 24 Parganas is the Sundarbans, a tidal region of islands, mangrove forests and creeks where the delta is still growing and the sea still enters with every tide. The Hooghly below Diamond Harbour is a tidal estuary with sandbanks that make navigation to Kolkata port difficult.

Thus one state contains the whole story of this section: the youthful river in the north, the mature river in the west and the old river with its delta in the south.

The work of rivers is the most examined part of the physical geography of the Madhyamik paper, and the questions follow a settled pattern that the student should know.

Objective questions (one mark) ask for names and terms: the process by which a river dissolves limestone (solution), the landform formed when a meander is cut off (oxbow lake), the delta type of the Mississippi (bird's-foot), the largest delta of the world (Ganga–Brahmaputra), the river that flows through a gorge at Sevoke (Teesta), the belt of coarse gravel at the foot of the Himalaya (Bhabar), the deposit formed where a river leaves the mountains (alluvial fan), the name of the river islands of the Brahmaputra (chars). True-or-false and fill-in-the-blank items test the same terms, so a list of about forty names from this section should be learnt with one line each.

Two-mark questions ask for a definition or a single difference: What is a pothole? What is meant by river capture? What is a natural levee? What is the difference between a tributary and a distributary? The answer should be two or three sentences with one example.

Three-mark questions ask the student to explain a formation with a diagram: How is a waterfall formed? How does an oxbow lake form? Why does the Teesta build a fan at Sevoke? The answer should give the cause, the process and the result in that order, followed by a labelled diagram.

Five-mark descriptive questions ask for a set of landforms or a comparison: Describe with diagrams three erosional landforms of a river; Describe the depositional landforms of the lower course; Explain the conditions favourable for the formation of a delta; Compare the work of a river in its three courses. These answers should be organised under sub-headings, one paragraph and one diagram per landform, and should include at least one Indian and one West Bengal example each.

Some points to remember while writing. Always state where a landform occurs (which course) and which process makes it (erosion or deposition) before describing its shape. Draw diagrams in pencil with clear labels; a waterfall diagram must show the hard and soft rock, the plunge pool and the direction of retreat; a meander diagram must show erosion on the outer bank and deposition on the inner bank; a delta diagram must show the distributaries. Use exact figures where you know them: 253 m for Jog Falls, about 1,00,000 km² for the Ganga delta. Use the technical terms of the chapter, such as hydraulic action, attrition, saltation, competence, flocculation, and give them a one-line meaning when first used. Keep the sequence of the chapter in mind, source to mouth, erosion to deposition, so that a long answer flows naturally and nothing important is left out.