Loktak lake

, image =
, caption = Different scenes of the Loktak lake of Manipur
, alt = View of Loktak Lake and Phumdis
, image_bathymetry =
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, location = Manipur
, coords =
, type = Fresh water ( lentic)
, inflow = Manipur River and many small rivulets
, outflow = Through barrage for hydropower generation, irrigation, and water supply
, catchment =
, basin_countries = India
, length =
, width =
, area = to
, depth =
, max-depth =
, volume =
, residence_time =
, shore =
, elevation =
, islands = Thanga, Ithing, Sendra islands. Also many floating islands called phumdis or phumshangs
, cities = Imphal & Moirang
, pushpin_map = India Manipur#India
, pushpin_map_alt = Location of lake in Manipur, India
, extra =

Loktak Lake ( mni, Loktak Pat) is a freshwater lake in Northeast India. It is the largest freshwater lake in South Asia along with Manchar Lake in Pakistan. It is a pulsating lake, with surface area varying from 250 sq km to 500 sq km during rainy season with a typical area of 287 sq km. The lake is located at Moirang in Manipur state, India. The etymology of ''Loktak'' is ''Lok'' = "stream" and ''tak'' = "the end" in Meitei language (Manipuri language). It is famous for the phumdi (heterogeneous mass of vegetation, soil and organic matter at various stages of decomposition) floating over it. The largest of all the phumdis covers an area of and is situated on the southeastern shore of the lake. Located on this phumdi, Keibul Lamjao National Park is the only floating national park in the world. The park is the last natural refuge of the endangered Sangai (state animal), ''Rucervus eldii eldii'' or Manipur brow-antlered deer (''Cervus eldi eldi''), one of three subspecies of Eld's deer.

Loktak Day is observed every year on the 15th of October at the periphery of the Loktak lake.

This ancient lake plays an important role in the economy of Manipur. It serves as a source of water for hydropower generation, irrigation and drinking water supply. The lake is also a source of livelihood for the rural fishermen who live in the surrounding areas and on phumdis, also known as "phumshangs". Human activity has led to severe pressure on the lake ecosystem. 55 rural and urban hamlets around the lake have a population of about 100,000 people.
Considering the ecological status and its biodiversity values, the lake was initially designated as a wetland of international importance under the Ramsar Convention on 23 March 1990. It was also listed under the ''Montreux Record'' on 16 June 1993, "a record of Ramsar sites where changes in ecological character have occurred, are occurring or are likely to occur".

Geography

Topography

Of the two river systems which drain the Manipur state—the Barak River and the Manipur River—the Manipur River flows through the Manipur Valley. The river rises in the north at Karong, flows south of Imphal and hence is also known as the Imphal River. Along its course through the valley downstream of Imphal, the bed slope of the river is gentle. The drainage pattern is sub-dendritic, sub-parallel and sub-radial, dictated by the structure and lithology of the area. Imphal, Iril and Thoubal River valleys also exhibit a meandering river course.

At the Ithai barrage outlet of the Lake, the direct catchment area draining into the lake is out of which is under paddy cultivation, under habitation and under forest cover. The area of the lake is , comprising large pockets of open water and marshy land formed at the southern part of the Imphal valley up to the confluence of Manipur River and Khuga River in the district of Imphal West. The Lake is divided into two zones, namely, the "Core Zone" which is the no-development zone or totally protected zone comprising and the balance area is called the "Buffer Zone".
Within the lake and on its periphery, there are 14 hills of varying size and elevation; in the southern part of the lake are the Sendra, Ithing and Thanga islands.

;Lake Zonation
Geographically, in terms of biodiversity and pressure of human activities, the lake is broadly divided into northern, central and southern zones as elaborated below.

The Northern Zone, extending from eastern side of Nambol River near Ngaikhong Khunou to Phabakchao including Maibam Phumlak, has five main streams/rivers namely the Manipur river, Nambul, Yangoi river, Nambol river and Thongjaorok flowing into the lake. The zone is separated from the central zone by large phumdis (varying thickness of ) that stretches from the northwest to the southeast. During January to March, phumdis in this area are usually burnt for construction of fish cum paddy farms; many large fish farms with raised levees have been built.

The Central Zone, extending from Awang Laisoi pat (western side of Nambol River near Ngaikhong Khullen) to Laphu pat (between Khordak channel and Imphal River) encloses prominent islands of Thanga, Karang and Ithing. It has some sub zone viz Laisoi, Awangsoi, Yennapat, Tharopat etc. It is the main open water zone of the lake, which was relatively free from phumdis in the past, but over the years 'athaphums' (artificially created phumdis for fishing) constructed by villagers for fishing have proliferated choking the entire lake. The State Fisheries Department has established a fisheries centre within this zone at Takmu pat for fisheries development

The Southern Zone encompasses the Keibul Lamjao National Park, Ungamel and Kumbi pats at the southern part of the lake and the zone is linked with Khuga River by the Ungamel channel. The Imphal River is also linked with this zone by the Khordak channel. The western catchment drained by the Kangshoibi River flows into this zone. Proliferation of phumdis has been observed near the mouth of Ungamel channel, Kumbi pat, Nongmaikhong and Khordak area.

Geology

Cretaceous limestone, the Disang with serpentinites (Lower to Middle Eocene–Upper Cretaceous), the Surmas and the Tipams (Miocene) are the dominant rock formations in the catchment of the Manipur basin. Higher reaches of hills have the Disang and Tipam groups of rocks while Surma group occurs in the lower reaches. Grey sandstone–grit–conglomerate–limestone sequences intruded by serpentinites containing minor amounts of enstalite, chromite, amphiboles and magnetite constitute the Disang formation. Argillaceous and arneceous sequence represents the Surma and Tipam groups, respectively.

Climate

With an average annual rainfall of tropical monsoon climate prevails in the valley. Temperatures range from 0 to 35 °C (32 to 95 °F). February and March are the driest months.

Hydrology

Loktak Lake is fed by the Manipur river and several tributaries and ‘Ungamel Channel’ (Ithai Barrage) is its only outlet now. The origin of the Manipur river system and its tributaries, which flows in a north-south direction and drains into the lake is from the hill ranges immediately to the west of the lake. The five major rivers with indirect catchment area of are the Imphal (also called the Manipur River), the Iril, the Thoubal, the Sekmai and the Khuga. The other major streams which drain into the lake and which bring in lot of silt are the Nambul, the Nambol, the Thongjarok, the Awang Khujairok, the Awang Kharok, the Ningthoukhong, the Potsangbam, the Oinam, the Keinou and the Irulok. The Lake, located on the southern side of the Manipur river basin, is at the lowest elevation in the valley and no other major river flows into the lake, except a few rivulets.

Though hydrological data on river basin has not been adequately monitored, the Department of Earth Science, Manipur University has in its report of 1996 assessed the average runoff of Manipur River as 519,200 ha·m (4,209,000 acre·ft; 5.192 km³) from a total catchment area of at the Ithai barrage. The ground water potential has been assessed estimated around per annum.

Water resources development

Several water resources development projects have been built in the Manipur River basin to meet growing demands for irrigation. Seven river valley projects have been completed; these are the Singda Dam Project, Thoubal Dam Project and Khuga Dam Project which are of multipurpose type, the Imphal Barrage Project and the Sekmai Barrage Project—both medium irrigation projects. The most discussed project is the Loktak Multipurpose Project, which provides hydropower, irrigation and water supply benefits but has attracted adverse criticism for the drastic alteration caused by the project to the hydrological regime of the Loktak Lake and associated wetlands.

Loktak Multipurpose Project

The Loktak Hydropower Project on the Manipur River or Imphal River, with the Loktak Lake forming the head waters to provide regulated storage for power generation, was built in 1983 as a multipurpose project with power generation of 105 MW (3x35 MW) for power supply to Manipur, Nagaland, Assam, Mizoram, Arunachal Pradesh, Meghalaya and Tripura, and lift irrigation to an area of in the Manipur valley. The main components of the project, as built, are, a) the Ithai Barrage, across the Manipur river of height and length with three spillway bays, b) water conductor system for trans-basin diversion comprising an open channel of length, a cut and cover section of length (bed width , followed by a Head Race Tunnel of diameter (horseshoe type) and long to carry a discharge of () for power generation and for irrigation), c) a surge shaft, d) penstocks and e) a surface powerhouse and a tailrace channel on the right bank of the narrow Leimatak River with three units of 35 MW capacity each generating an annual energy of 448 GW·h (1.613 PJ). The downstream Loktak Power Project in cascade to utilize the regulated releases from the upper project for further power generation of 90 MW is proposed to be taken up for joint implementation by NHPC and the Government of Manipur.
The project has invited severe criticism since the water level in the lake at the Ithai barrage is maintained throughout the year at FRL from considerations of power generation from the well planned water conductor system. The problem in the lake is, however, diverse; economic activities in and around the lake, and encroachments by 50,400 fishermen for extensive fishing activity are also cited to be some reasons. Other reasons for the distress situation in the lake are a) jhum (shifting) cultivation, b) extensive deforestation and unscientific land use practices in the catchment areas which bring large amount of silt into the lake and c) undefined ownership of the lake where the stakeholders are the districts of Imphal west district, Bishnupur and Thoubal causing lack of integrated approach to the lakes potential for providing benefits.

Biodiversity

A rich biodiversity with habitat heterogeneity has been recorded during a scientific survey carried out between January 2000 and December 2002 in different habitat patches of the lake. The lake’s rich biological diversity comprises 233 species of aquatic macrophytes of emergent, submergent, free-floating and rooted floating leaf types. But in the declining trend of vegetation, the important flora recorded are as under.

Vegetation

The important vegetation of the phumdis recorded are '' Eichhornia crassipes'', ''Phragmites karka'', '' Oryza sativa'', ''Zizania latifolia'', '' Cynodon'' spp., '' Limnophila'' spp., ''Sagittaria