Chapter 3 Drainage System

The flow of water through well-defined channels on the Earth's surface is called drainage. The interconnected network of these channels forms a drainage system. A drainage pattern in a specific area is shaped by various factors, including the geological history, the type and structure of the rocks, the topography and slope of the land, and the volume and seasonality of water flow.

Rivers flow in a specific direction primarily due to the slope of the land, following gravity from higher elevations to lower ones. For example, rivers originating in the Himalayas in the north and the Western Ghats in the south generally flow eastward towards the Bay of Bengal because the overall slope of the Peninsular plateau is tilted eastward.

The area from which a river collects its water is called its catchment area. The entire area drained by a river and its tributaries constitutes a drainage basin or river basin (Figure 3.1 shows a river in a mountainous region, part of a drainage basin). The dividing line that separates one drainage basin from another is known as a watershed or drainage divide.

Image showing a river channel carving its way through a mountainous landscape, part of a river's drainage system and basin.

River basins are generally larger areas drained by major rivers, while watersheds are smaller areas drained by minor streams and rivulets. Despite the size difference, river basins and watersheds are considered unified units. Actions in one part directly affect other parts and the whole system. This makes them useful units for planning and resource management at micro, meso, or macro levels.

Drainage System

Important Drainage Patterns

The arrangement and form of the network of river channels in a drainage basin create distinct patterns:

• Dendritic: Resembling the branches of a tree. This pattern develops where the underlying rock structure is uniform and streams flow over homogeneous material, following the general slope. Rivers of the northern plains of India (Ganga, Indus, Brahmaputra) exhibit dendritic patterns in their vast basins.
• Radial: Channels flow outwards in all directions from a central elevated point like a hill or volcano. Rivers originating from the Amarkantak range (like Narmada, Son) show a radial pattern.
• Trellis: A rectangular pattern where primary tributaries flow roughly parallel to each other, and secondary tributaries join them at nearly right angles. This pattern often develops in areas with folded topography and alternating bands of resistant and less resistant rocks.
• Centripetal: Streams converge from all directions into a central depression or lake, where the water accumulates without flowing out to the sea.

The Indian drainage system can be classified based on the direction of water discharge into the sea:

• Arabian Sea Drainage: Rivers that discharge their waters into the Arabian Sea (about 23% of India's drainage area). Includes the Indus, Narmada, Tapi, Mahi, Periyar, and numerous smaller west-flowing rivers.
• Bay of Bengal Drainage: Rivers that discharge their waters into the Bay of Bengal (about 77% of India's drainage area). Includes the Ganga, Brahmaputra, Mahanadi, Krishna, Godavari, Kaveri, and numerous smaller east-flowing rivers.

These two major drainage divisions are separated by a water divide formed by the Delhi ridge, the Aravali range, and the Sahyadris (Western Ghats) (Figure 3.1 indicates this water divide). While most large Peninsular rivers originate in the Western Ghats and flow east, the Narmada and Tapi are significant exceptions that flow westward into the Arabian Sea through rift valleys.

Based on the size of the drainage basin (catchment area), India's river basins are categorized:

• Major River Basins: Catchment area > 20,000 sq. km (14 basins, e.g., Ganga, Indus, Brahmaputra, Godavari, Krishna, Narmada).
• Medium River Basins: Catchment area between 2,000-20,000 sq. km (44 basins, e.g., Periyar, Meghna).
• Minor River Basins: Catchment area < 2,000 sq. km (a considerable number of rivers, often in low-rainfall areas).

Another common classification, based on origin, nature, and characteristics, divides India's drainage into:

• The Himalayan Drainage
• The Peninsular Drainage

Although this classification is not perfect (some rivers originating in the northern Peninsula join the Ganga system), it is widely used and adopted in this chapter.

Drainage Systems Of India

The Indian drainage system is extensive, comprising many small and large rivers. Its development is closely tied to the geological evolution of India's major physiographic divisions and influenced by the pattern and characteristics of precipitation.

The Himalayan Drainage

The Himalayan drainage system consists of rivers originating in the Himalayas and the Trans-Himalayan region. It primarily includes the river systems of the Ganga, the Indus, and the Brahmaputra. These rivers have a long geological history and are characterized by being perennial – they have water throughout the year. This is because they are fed by the melting of Himalayan glaciers and snow (snow melt) and also receive rainfall during the monsoon season.

Himalayan rivers flow through deep gorges that they have carved as the Himalayas were uplifted, indicating their antecedent nature (existing before the mountain ranges fully formed and cutting through them as they rose). In their mountainous courses, they exhibit features of a youthful stage of development, such as V-shaped valleys, rapids (Figure 3.3), and waterfalls.

Image showing rapids, sections of a river with turbulent flow and white water, typically found in steep, rocky channels.

Upon entering the plains, these rivers slow down and deposit vast amounts of sediment, forming depositional landforms characteristic of a mature stage, including flat floodplains, ox-bow lakes (formed from cut-off meanders), braided channels (where the river channel splits into multiple interwoven smaller channels), and large deltas at their mouths where they meet the sea.

In the mountainous regions, the river courses can be very sinuous (tortuous), but in the plains, they develop strong meandering patterns and are known for frequently shifting their courses, particularly during floods. The Kosi river, often called the 'sorrow of Bihar', is notorious for its frequent course changes, primarily because it carries huge quantities of sediment from its upper reaches, which are deposited in the plains, blocking the channel and forcing the river to find a new path. The discharge of these rivers fluctuates throughout the year, being highest during the monsoon season due to rainfall and snowmelt.

Flooding, while essential for replenishing floodplains and maintaining ecosystems, can also cause significant damage to life, property, and infrastructure (negative effect).

Evolution Of The Himalayan Drainage

There are varying geological theories regarding the exact evolution of the Himalayan drainage. One widely discussed theory suggests that a massive, unified river system, often referred to as the Shiwalik or Indo-Brahma river, flowed longitudinally along the entire extent of the Himalayas from Assam to Punjab and onward to Sindh (now in Pakistan), discharging into the Gulf of Sindh, around 5 to 24 million years ago (during the Miocene period). Evidence supporting this includes the geological continuity of the Shiwalik range, which formed from the deposits of this large river, and the presence of lacustrine (lake) and alluvial deposits (sands, silts, clays, boulders) throughout the Shiwalik foothills.

This hypothetical Indo-Brahma river system is believed to have been fragmented into the current major drainage systems (Indus, Ganga, Brahmaputra) due to geological events in the Pleistocene epoch (starting about 2.6 million years ago). A major upheaval in the western Himalayas, including the uplift of the Potwar Plateau (which extends to the Delhi Ridge), acted as a water divide, separating the Indus and Ganga systems. Later, a down-faulting event in the Malda Gap region (between the Rajmahal Hills and the Meghalaya Plateau) diverted the Ganga and Brahmaputra river systems eastward towards the Bay of Bengal, creating the current drainage orientation.

The River Systems Of The Himalayan Drainage

The Himalayan drainage is dominated by three major river systems:

The Indus System

The Indus river system is one of the largest in the world, with a total length of 2,880 km (1,114 km in India) and a vast basin area (321,289 sq. km in India). Also known as the Sindhu, it is the westernmost of the major Himalayan rivers in India. It originates from a glacier near Bokhar Chu (4,164 m altitude) in the Kailash range of the Tibetan region (where it is called 'Singi Khamban' or Lion's Mouth). It flows northwest between the Ladakh and Zaskar ranges, passing through Ladakh and Baltistan. It carves a spectacular gorge near Gilgit before entering Pakistan near Chilas.

Important tributaries joining the Indus include numerous Himalayan rivers from the north (Shyok, Gilgit, Zaskar, Hunza, Nubra, Shigar, Gasting, Dras) and rivers originating from the Sulaiman ranges to its right (Khurram, Tochi, Gomal, Viboa, Sangar). South of Mithankot in Pakistan, the Indus receives the 'Panjnad', which is the combined flow of the five rivers of Punjab: Satluj, Beas, Ravi, Chenab, and Jhelum. The Indus finally discharges into the Arabian Sea east of Karachi. Within India, the Indus river itself flows only through the Union Territory of Jammu and Kashmir (now bifurcated into UTs of J&K and Ladakh).

• Jhelum: Rises from a spring at Verinag in the southeastern Kashmir valley. It flows through Srinagar and Wular Lake before entering Pakistan through a gorge and joining the Chenab near Jhang.
• Chenab: The largest tributary of the Indus. It is formed by the confluence of two headstreams, the Chandra and the Bhaga, at Tandi in Himachal Pradesh, hence also called Chandrabhaga. It flows for 1,180 km before entering Pakistan and joining the Jhelum.
• Ravi: Originates west of the Rohtang Pass in Himachal Pradesh's Kullu hills and flows through the Chamba valley. It drains the area between the Pir Panjal and Dhauladhar ranges before joining the Chenab in Pakistan.
• Beas: Rises from Beas Kund near the Rohtang Pass (4,000 m). It flows through the Kullu valley and forms gorges in the Dhauladhar range before entering the Punjab plains and meeting the Satluj near Harike.
• Satluj: Originates from Raksas Tal near Mansarovar Lake in Tibet (known as Langchen Khambab). It flows parallel to the Indus for about 400 km before entering India through the Shipki La pass and cutting a gorge at Rupar in the Punjab plains. It is an antecedent river (existed before the Himalayan uplift). The Satluj is crucial for the Bhakra Nangal project's canal system.

The Ganga System

The Ganga is India's most significant river, culturally and for its vast basin (8.6 lakh sq. km in India). It rises from the Gangotri glacier near Gaumukh (3,900 m) in Uttarakhand, where it is initially known as the Bhagirathi. The Bhagirathi flows through gorges in the Himalayas and meets the Alaknanda at Devprayag; from this confluence, the river is named the Ganga. The Alaknanda originates from the Satopanth glacier above Badrinath and is formed by the union of the Dhauli and Vishnu Ganga at Joshimath (Vishnu Prayag). Other Alaknanda tributaries are the Pindar (joining at Karna Prayag) and Mandakini/Kali Ganga (joining at Rudra Prayag). The Ganga enters the plains at Haridwar.

From Haridwar, the Ganga flows south, then southeast and east, eventually splitting into two distributaries: the Bhagirathi (in West Bengal) and the Padma (entering Bangladesh). The Ganga has a total length of 2,525 km within India, flowing through Uttarakhand, Uttar Pradesh, Bihar, and West Bengal.

The Ganga basin is the largest in India, receiving numerous perennial tributaries from the Himalayas (left bank) and non-perennial tributaries from the Peninsula (right bank). The major right bank tributary is the Son. Important left bank tributaries from the Himalayas include the Ramganga, Gomati, Ghaghara, Gandak, Kosi, and Mahananda. The Ganga finally discharges into the Bay of Bengal near Sagar Island.

• Yamuna: The westernmost and longest tributary of the Ganga, originating from the Yamunotri glacier. It joins the Ganga at Prayag (Allahabad). Its right bank tributaries (from the Peninsula) include the Chambal, Sind, Betwa, and Ken. Left bank tributaries include the Hindan, Rind, Sengar, Varuna. Much of its water is used for irrigation via canals (Western Yamuna, Eastern Yamuna, Agra canals).
• Chambal: Rises near Mhow in the Malwa plateau. It flows northwards through a gorge near Kota (site of Gandhisagar dam) and then passes through Bundi, Sawai Madhopur, and Dholpur before joining the Yamuna. The Chambal is known for its badland topography and ravines.
• Gandak: Formed by the streams Kaligandak and Trishulganga in the Nepal Himalayas. It drains central Nepal and enters the Ganga plain in Bihar, joining the Ganga near Sonpur.
• Ghaghara: Originates from glaciers near Mapchachungo. It receives tributaries Tila, Seti, and Beri, cuts a gorge at Shishapani, and is joined by the Sarda (Kali Ganga) in the plains before meeting the Ganga at Chhapra.
• Kosi: An antecedent river rising north of Mount Everest in Tibet (main stream Arun). It is joined by the Son Kosi (west) and Tamur Kosi (east) in Nepal, forming the Sapt Kosi before entering India. Known as the 'sorrow of Bihar' due to frequent course changes caused by heavy sediment load.
• Ramganga: A smaller river rising in the Garhwal hills. It flows southwest after crossing the Shiwaliks and joins the Ganga near Kannauj in Uttar Pradesh.
• Damodar: Flows through a rift valley in the eastern Chotanagpur Plateau, joining the Hugli (a distributary of the Ganga). Its main tributary is the Barakar. Formerly known as the 'sorrow of Bengal' due to floods, it is now controlled by the Damodar Valley Corporation project.
• Sarda (Saryu/Goriganga): Rises in the Milam glacier in the Nepal Himalayas. Known as Kali or Chauk along the Indo-Nepal border, it joins the Ghaghara in the plain.
• Mahananda: Rises in the Darjiling hills and is the Ganga's last left bank tributary in West Bengal.
• Son: A large south bank tributary originating in the Amarkantak plateau. It flows north, forming waterfalls at the plateau edge, and joins the Ganga near Arrah, west of Patna.

The Indian government has launched the 'Namami Gange Programme' in 2014 as an Integrated Conservation Mission for the river Ganga, focusing on pollution abatement, conservation, and rejuvenation.

The Brahmaputra System

The Brahmaputra is one of the world's largest rivers, originating from the Chemayungdung glacier near Mansarovar Lake in the Kailash range. In Tibet, it flows eastward for about 1,200 km through a flat, dry region, known as the 'Tsangpo' (the purifier). Its major right bank tributary in Tibet is the Rango Tsangpo. The Brahmaputra then turns south, carving a deep gorge through the Himalayas near Namcha Barwa (7,755 m), emerging from the foothills in Arunachal Pradesh under the name 'Siang' or 'Dihang'.

It enters India west of Sadiya town in Arunachal Pradesh. Flowing southwest through the Assam valley (about 750 km), it receives major left bank tributaries like the Dibang (Sikang) and Lohit, after which it is officially known as the Brahmaputra. Important right bank tributaries are the Subansiri (an antecedent river originating in Tibet), Kameng, Manas, and Sankosh. Major left bank tributaries in Assam include the Burhi Dihing and Dhansari (South).

The Brahmaputra enters Bangladesh near Dhubri, flowing southward as the Jamuna (after being joined by the Tista from the right). It finally merges with the Padma (main arm of the Ganga) and discharges into the Bay of Bengal. The Brahmaputra is known for its high flood potential, frequent channel shifting, and bank erosion, largely due to the large volume of water and sediment contributed by its numerous tributaries, particularly during the heavy monsoon rains in its catchment area.

The Peninsular Drainage System

The Peninsular drainage system is considered geologically older than the Himalayan one. Evidence for its maturity includes the presence of broad, shallow, and largely graded river valleys. The Western Ghats, running parallel to the west coast, serve as the main water divide for Peninsular rivers. Most major rivers originate here and flow east into the Bay of Bengal, while smaller, short rivers flow west into the Arabian Sea. Most Peninsular rivers have a fixed course, generally lack meanders (except in limited areas), and are non-perennial, meaning their flow is dependent on seasonal rainfall and fluctuates significantly, often drying up during the dry season (except for the Narmada and Tapi, which flow through rift valleys and maintain some flow).

Some northern Peninsular rivers like the Chambal, Sind, Betwa, Ken, and Son, which are older than the Himalayas, are part of the Ganga river system.

The Evolution Of Peninsular Drainage System

The current pattern of Peninsular drainage is attributed to three major geological events:

• Subsidence of the Western Flank: In the early Tertiary period (beginning about 66 million years ago), the western part of the Peninsula subsided and was submerged beneath the sea. This tilted the overall plateau eastward, disrupting the symmetrical drainage that might have existed and orienting most rivers towards the east.
• Himalayan Upheaval and Trough Faulting: The uplift of the Himalayas in the north subjected the northern flank of the Peninsular block to stress, causing subsidence and faulting (trough faulting). The Narmada and Tapi rivers flow in these fault troughs (rift valleys), which prevented the accumulation of large alluvial or deltaic deposits at their mouths like the east-flowing rivers.
• Tilting of the Peninsular Block: A slight overall tilting of the Peninsular block from northwest to southeast also occurred, reinforcing the eastward drainage pattern towards the Bay of Bengal.

River Systems Of The Peninsular Drainage

Major river systems flowing eastward into the Bay of Bengal:

• Mahanadi: Rises near Sihawa in Chhattisgarh. Length 851 km, catchment area 1.42 lakh sq. km. Flows through Odisha and forms a delta before reaching the Bay of Bengal.
• Godavari: The largest Peninsular river system, also called Dakshin Ganga. Rises in the Nasik district of Maharashtra. Length 1,465 km, catchment area 3.13 lakh sq. km. Flows through Maharashtra, MP, Chhattisgarh, Odisha, and Andhra Pradesh, forming a large delta near Rajamundri. Principal tributaries: Penganga, Indravati, Pranhita, Manjra. Experiences floods in lower reaches.
• Krishna: Second largest east-flowing Peninsular river. Rises near Mahabaleshwar in the Sahyadri (Western Ghats). Length 1,401 km, catchment area 2.58 lakh sq. km. Major tributaries: Koyna, Tungbhadra, Bhima. Flows through Maharashtra, Karnataka, and Andhra Pradesh/Telangana.
• Kaveri: Rises in Brahmagiri hills (1,341 m) in Karnataka. Length 800 km, drainage area 81,155 sq. km. Unique for having water throughout the year with less fluctuation than other Peninsular rivers because its upper catchment receives southwest monsoon rain (summer) and its lower part receives northeast monsoon rain (winter). Important tributaries: Kabini, Bhavani, Amravati. Flows through Kerala, Karnataka, and Tamil Nadu.

Major river systems flowing westward into the Arabian Sea:

• Narmada: Originates on the western flank of the Amarkantak plateau (1,057 m). Flows westward in a rift valley between the Satpura and Vindhyan ranges. Forms a picturesque gorge in marble rocks and Dhuandhar waterfall near Jabalpur. Length 1,312 km, catchment area 98,796 sq. km. Forms a 27 km long estuary south of Bharuch. The Sardar Sarovar Project is built on this river. (The Indian government also has a conservation mission for the Narmada called "Namami Devi Narmade").
• Tapi: Rises from Multai in the Betul district of MP. Length 724 km, drainage area 65,145 sq. km. Flows westward in a rift valley parallel to the Narmada. Most of its basin is in Maharashtra, followed by MP and Gujarat.

Largest river system of Rajasthan (west of Aravali):

• Luni: Rises near Pushkar from the Saraswati and Sabarmati streams joining at Govindgarh. Flows west through the Aravali range, then southwest to the Rann of Kachchh, where it disappears into the marshy land. It is an ephemeral river system, characteristic of inland drainage in arid regions.

Smaller Rivers Flowing Towards The West

Numerous smaller rivers flow westwards into the Arabian Sea, mainly originating in the Western Ghats and having short courses due to the steep western slope and the proximity of the water divide to the coast.

Examples in Gujarat include Shetruniji, Bhadra, Dhadhar, Sabarmati, and Mahi. Important west-flowing rivers in Maharashtra include Vaitarna (rises from Trimbak hills). In Karnataka, prominent west-flowing rivers include Kalinadi, Bedti, and Sharavati (originates in Shimoga, drains 2,209 sq km). The Gersoppa (Jog) Falls are on the Sharavati river. Goa has two notable west-flowing rivers: Mandovi and Juari.

Kerala has a very narrow coastline, and its west-flowing rivers are relatively short. The longest river is Bharathapuzha (also Ponnani), rising near Annamalai hills, draining 5,397 sq km. The Periyar is the second largest (catchment area 5,243 sq km). The Pamba river (177 km) falls into the Vemobanad lake.

Comparison of catchment areas for some west-flowing rivers:

Most of these smaller west-flowing rivers are important for local water supply, irrigation, and hydropower.

Small Rivers Flowing Towards The East

Along with the major east-flowing rivers (Mahanadi, Godavari, Krishna, Kaveri), there are also numerous smaller rivers that flow eastward into the Bay of Bengal. Though smaller in size and basin area, they are locally significant.

Important smaller east-flowing rivers include Subarnarekha, Baitarni, Brahmani, Vamsadhara, Penner, Palar, and Vaigai.

Comparison of catchment areas for some smaller east-flowing rivers:

These rivers are important for local and regional water supply and irrigation in the eastern coastal plains.

River Regimes

The regime of a river refers to the pattern of flow of water in the river channel throughout the year. It describes how the volume of water discharged by the river varies seasonally. River discharge is the volume of water flowing past a specific point in a unit of time, typically measured in cusecs (cubic feet per second) or cumecs (cubic meters per second).

Comparison between the Himalayan and Peninsular River Regimes (Table 3.1):

Himalayan rivers, being fed by both snowmelt and rainfall, maintain a relatively high flow throughout the year, with peak flow during the monsoon season and significant flow during summer due to glacial melt. Peninsular rivers are mainly rain-fed; their flow is seasonal, increasing significantly during the monsoon and decreasing sharply or drying up during the dry season. The regime of Peninsular rivers is highly dependent on the varying rainfall patterns across the plateau.

Examples of river regimes:

• Ganga: Experiences minimum flow during the lean period (January-June) before the monsoon. Maximum flow is reached during the peak monsoon months (August-September). The Ganga maintains a sizeable flow in early summer due to snowmelt from the Himalayas. The difference between its mean maximum discharge during the monsoon (around 55,000 cusecs at Farakka) and mean minimum discharge in the dry season (only 1,300 cusecs) is large, reflecting the impact of monsoon rainfall and glacial melt.
• Narmada: A Peninsular river flowing westward. Shows very low discharge from January to July. A sudden rise in flow occurs in August, reaching maximum discharge (around 2,300 cusecs at Garudeshwar). The flow drops quickly in October. Minimum flow (only 15 cusecs) is during the dry season.
• Godavari: An east-flowing Peninsular river. Minimum discharge in May. Maximum flow in July-August due to monsoon rains. Sharp fall after August, but flow in October-November is still higher than in the dry months (January-May). Mean maximum discharge around 3,200 cusecs at Polavaram, mean minimum 50 cusecs.

These examples illustrate how Himalayan rivers have a more sustained flow due to snowmelt, while Peninsular rivers have marked seasonal fluctuations driven by monsoon rainfall. The specific timing and magnitude of peak flow vary between regions depending on the onset and intensity of the monsoon and the contribution of snowmelt.

Extent Of Usability Of River Water

India's rivers carry a large volume of water annually, but this water is distributed unevenly in time and space. This leads to paradoxes: perennial rivers have water year-round, while non-perennial rivers have very little during dry seasons. During the monsoon, excessive water in some rivers causes floods, while other areas simultaneously suffer from droughts. This highlights not just a problem of water availability but also a challenge of water resource management.

Possible measures to mitigate simultaneous floods and droughts include:

• Improved water storage infrastructure (dams, reservoirs) to store surplus floodwater for use during dry periods.
• Efficient water distribution systems (canals, pipelines).
• Rainwater harvesting and watershed management in drought-prone areas.
• Inter-basin water transfer schemes (linking rivers) to move surplus water from flood-prone basins to deficit basins.

India has implemented some inter-basin linkages, such as the Periyar Diversion Scheme, Indira Gandhi Canal Project (linking Satluj-Beas waters to Rajasthan desert), Kurnool-Cuddapah Canal (linking Krishna water), and Beas-Satluj Link Canal. Proposed large-scale Ganga-Kaveri link canal projects aim to transfer water from Himalayan rivers to Peninsular rivers.

Inter-linking rivers, especially between the Northern Plain and the Peninsular Plateau, presents significant challenges:

• Topography: The Peninsular Plateau is at a higher elevation than the Northern Plain. Transferring water from the plains to the plateau requires significant energy for pumping, or reliance on gravity flow across carefully planned routes.
• Water Availability: There are debates about whether northern rivers actually have sufficient *surplus* water, especially during the dry season, that can be reliably transferred to other basins without impacting their own ecosystems, irrigation, and downstream needs (including maintaining flow to the sea).
• Environmental Impacts: Large-scale construction and alteration of river flows can have significant negative ecological and social consequences.
• Inter-state Disputes: Water sharing is a sensitive issue, leading to disputes between states over water allocation and transfer.

Ranking problems in using river water:

1. River water pollution
2. Uneven seasonal flow of water
3. River water disputes between states
4. No availability in sufficient quantity (often related to seasonal flow and regional distribution)
5. Load of silt in the river water
6. Shrinking of channels due to extension of settlements (encroachment on floodplains/channels)

Rivers are polluted by various sources, including: discharge of untreated sewage from cities, disposal of industrial effluents and wastes, traditional practices like cremation on river banks and disposing of dead bodies, and large-scale bathing and washing. Cleaning up rivers requires comprehensive measures like setting up sewage treatment plants, controlling industrial pollution, regulating activities on river banks, and promoting public awareness and responsible waste disposal. Initiatives like the Ganga Action Plan and campaigns to clean the Yamuna are examples of efforts to reduce river pollution in India.

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