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Mullaperiyar Dam () is a masonry gravity dam on the Periyar River of Idukki district of Indian state of Kerala. It is situated 150 km south east of Kochi and 200 km north east of state capital city of Trivandrum. It is located above the sea level, on the Cardamom Hills of the Western Ghats in Thekkady, Idukki District of Kerala, India. It was constructed between 1887 and 1895 by John Pennycuick and also reached in an agreement to divert water eastwards to the Madras Presidency area (present-day Tamil Nadu). It has a height of from the foundation, and a length of . The Periyar National Park in Thekkady is located around the dam's reservoir. The dam is built at the confluence of Mullayar and Periyar rivers. The dam is located in Kerala on the river Periyar, but is operated and maintained by the neighbouring state of Tamil Nadu. Although the Periyar River has a total catchment area of 5398 km2 with 114 km2 downstream from the dam in Tamil Nadu, the catchment area of the Mullaperiyar Dam itself lies entirely in Kerala and thus not an inter-State river. On 21 November 2014, the water level hit 142 feet for first time in 35 years. The reservoir again hit the maximum limit of 142 feet on 15 August 2018, following incessant rains in the state of Kerala. In a 2021 UNU-INWEH report about ageing large dams around the world, Mullaperiyar dam was said to be "situated in a seismically active area with significant structural flaws and poses risk to 5-10 million people if the 130+ years old dam were to fail".
Currently, the water from the Periyar (Thekkady) Lake created by the dam, is diverted through the water shed cutting and a tunnel to Forebay Dam near Kumily (Iraichalpalam). From the Forebay dam, hydroelectricity pipelines carry the water to the Periyar Power Station in Lower Periyaru. This is used for power generation (180 MW capacity) in the Periyar Power Station. From the Periyar Power Station, the water is let out into Vairavanar river and then to Suruliyar and from Suruliyar to Vaigai Dam.
The first attempt at damming the Periyaar with an earthen dam in 1850 was given up due to demands for higher wages by the labour citing unhealthy living conditions. The proposal was resubmitted a number of times and in 1862, Captain J. G. Ryves, M.E., carried out a study and submitted proposals in 1867 for another earthwork dam, 62 feet high. The matter was debated by the Madras Government and the matter further delayed by the terrible famine of 1876–77. Finally, in 1882, the construction of the dam was approved and Major John Pennycuick, M.E., placed in charge to prepare a revised project and estimate which was approved in 1884 by his superiors.
In 1947, after Indian Independence, Travancore and Cochin joined the Union of India and on 1 July 1949 were merged to form Travancore-Cochin. On 1 January 1950, Travancore-Cochin was recognised as a state. The Madras Presidency was organised to form Madras State in 1947. On 1 November 1956, the state of Kerala was formed by the States Reorganisation Act. The Kerala state government announced that the earlier agreement which had been signed between British Raj and Travancore agreement was invalid and needed to be renewed.
After several failed attempts to renew the agreement in 1958, 1960, and 1969, the agreement was renewed in 1970 when C Achutha Menon was Kerala Chief Minister. According to the renewed agreement, the tax per acre was increased to 30, and for the electricity generated in Lower Camp using Mullaperiyar water, the charge was 12 per kilowatt-hour. Tamil Nadu uses the water and the land, and the Tamil Nadu government has been paying to the Kerala government for the past 50 years 2.5 as tax per year for the whole land and 7.5 lakhs per year as surcharge for the total amount of electricity generated. The validity of this agreement is under dispute between the States of Kerala and Tamil Nadu. As of 2013 the matter is pending before a Division Bench of the Supreme Court. The dispute puts into question the power of the federal government of India to make valid orders respecting Indian States, in this case regarding a watershed and dam within one state that is used exclusively in another.
The construction involved the use of troops from the 1st and 4th battalions of the Madras Pioneers as well as Portugal carpenters from Cochin who were employed in the construction of the coffer-dams and other structures. The greatest challenge was the diversion of the river so that lower portions of the great dam could be built. The temporary embankments and coffer-dams used to restrain the river waters were regularly swept away by floods and rains. Due to the coffer dam failures, the British stopped funding the project. Officer Pennycuick raised funds by selling his wife's jewelry to continue the work. In Madurai, Pennycuick's statue has been installed at the state PWD office and his photographs are found adorning walls in peoples homes and shops. In 2002, his great-grandson was honoured in Madurai, a function that was attended by thousands of people.
The dam created a reservoir in a remote gorge of the Periyar river situated 3,000 feet above the sea in dense and malarial jungle, and from the northerly arm of this manmade waterbody, the water flowed first through a deep cutting for about a mile and then through a tunnel, 5704 feet in length and later through another cutting on the other side of the water divide and into a natural ravine and so onto the Vaigai River which has been partly built up for a length of 86 miles, finally discharging 2000 cusecs of water for the arid rain shadow regions of present-day Theni, Dindigul District, Madurai District, Sivaganga District and Ramanathapuram districts of Tamil Nadu, then under British rule as part of Madras Province (Sandes, 1935).
The Periyar project, as it was then known, was widely considered well into the 20th century as "one of the most extraordinary feats of engineering ever performed by man". A large amount of manual labour was involved and worker mortality from malaria was high. It was claimed that had it not been for "the medicinal effects of the native spirit called arrack, the dam might never have been finished". Disease killed 483 people during the construction of this dam and were buried on-site in a cemetery just north of the dam.
In 2012, it was announced that a memorial dedicated to dam engineer Pennycuick would be erected at the dam site.
Other mammals found here include gaur, bison, sambar deer (horse deer), barking deer, mouse deer, (Indian wild dogs), mongoose, and . Four species of primates are found at Periyar – the rare lion-tailed macaque, the Nilgiri langur, the common langur, and the bonnet macaque. According to a report by the Kerala Forest Research Institute, the protected area surrounding the dam and reservoir is classified as a biodiversity hot spot.
Strengthening measures adopted by Tamil Nadu PWD from 1979 with the recommendation of Central Water Commission (CWC) include grouting of the old dam with concrete, guniting the upstream face of dam, cable anchoring of the dam's structure with the foundation throughout its length, RCC(Reinforced Concrete Construction) capping on top of the dam at height above 145 feet, building 10 meter RCC concrete backing structure with 10 feet foundation up to height of 145 feet in downstream face of dam and binding the RCC structure using shear keys and concrete grouting with the old structure and the foundation along the downstream face of dam, building additional drainage galleries in order to measure seepage and additional sluice gates for water evacuation.
During a 2011 scanning of the Mullaperiyar dam using a remotely operated vehicle by the Central Soil and Materials Research Station on directions from the Empowered Committee of the Supreme Court, the Kerala Government observer opined that "mistakes in the strengthening works carried out by Tamil Nadu" in 1979 damaged the masonry of the dam.
Current safety concerns relate to several issues. Since the dam was constructed using stone rubble masonry with lime mortar grouting following prevailing 19th-century construction techniques that have now become archaic, seepage and leaks from the dam have caused concern. Moreover, the dam is situated in a seismically active zone. An earthquake measuring 4.5 on the Richter scale occurred on 7 June 1988 within 20 km of the dam. Subsequently, several tremors have occurred in the area in recent times. These could be reservoir-induced seismicity, requiring further studies according to experts. A 2009 report by IIT Roorkee stated that the dam "was likely to face damage if an earthquake of the magnitude of 6.5 on the Richter scale struck its vicinity when the water level is at 136 feet".
The then late Tamil Nadu Chief Minister M. Karunanidhi said that immediately after the Supreme Court announced its decision to set up a committee, he had written to Congress president asking the centre to mediate between Kerala and Tamil Nadu on Mullaperiyar issue. However, the then Leader of Opposition i.e., the late Chief Minister of Tamil Nadu J. Jayalalithaa objected to the TN Government move. She said that this would give advantage to Kerala in the issue. Meanwhile, Kerala Water Resources Minister N. K. Premachandran told the state Assembly that the State should have the right of construction, ownership, operation and maintenance of the new dam, while giving water to Tamil Nadu on the basis of a clear cut agreement. He also informed the media that former Supreme Court Judge K. T. Thomas will represent Kerala on the expert panel constituted by Supreme Court.
On 8 March 2010, Tamil Nadu told the Supreme Court that it was not interested in adjudicating the dispute with Kerala before the special "empowered" committee appointed by the apex court for settling the inter-State issue. However, Supreme Court refused to accept Tamil Nadu's request to scrap the decision to form the empowered committee. The Supreme Court also criticized the Union Government on its reluctance in funding the empowered committee.
The Empowered Committee headed by the former Chief Justice of India A.S. Anand, in its findings concluded that the dam is "structurally, hydrologically safe, and Tamil Nadu can raise the water level from 136 to 142 feet after carrying out repairs."
Tamil Nadu has insisted on raising the water level in the dam to 142 feet, pointing out crop failures. However, the court has clarified Tamil Nadu is not a riparian state. "In the process the farmers of the erstwhile rain shadow areas in Tamil Nadu who had started a thrice yearly cropping pattern had to go back to the bi-annual cropping."
The Kerala Government maintains that this is not true. During the year 1979–80 the gross area cultivated in Periyar command area was . After the lowering of the level to , the gross irrigated area increased and in 1994–95 it reached . Kerala Government's arguments . Expert-eyes.org. Retrieved 25 November 2011.
In 2006, the Supreme Court of India by its decision by a three-member division bench, allowed for the storage level to be raised to pending completion of the proposed strengthening measures, provision of other additional vents and implementation of other suggestions.
However, the Kerala Government promulgated a new "Dam Safety Act" against increasing the storage level of the dam, which has been challenged by Tamil Nadu on various grounds. The Supreme Court issued notice to Kerala to respond, however did not stay the operation of the Act even as an interim measure. The Court then advised the States to settle the matter amicably, and adjourned hearing in order to enable them to do so. The Supreme Court of India termed the act as not unconstitutional. Mullaperiyar: apex court issues notice to Government . The Hindu. 9 September 2006. Retrieved 25 November 2011. Meanwhile, the Supreme Court constituted a Constitution bench to hear the case considering its wide ramifications.
The Kerala Government states that it does not object to giving water to Tamil Nadu as a shared commodity, their main cause of objection being the dam's safety due to its age. Increasing the level would add more pressure to be handled by the already leaking dam. They also raised the problematic definition of inter-State river when in its entirety the river goes into the sea within Kerala without passing through any other states. They also raised the complexity of the judgment and disputes that it can raise in other states that have small catchments of other-State rivers and right to claim part of waters.
In May 2014 the Supreme Court of India declared the Kerala Irrigation and Water Conservation (Amendment) Act of 2006 as unconstitutional and struck down the law passed by the Kerala Assembly on the Mullaperiyar Dam that said that the water level cannot be increased beyond 136 feet. The court has ruled that Tamil Nadu can increase the water level to 142 feet and constituted a permanent Supervisory Committee in order to take care of the affairs relating to maintenance of the Mullaperiyar Dam.
To assist the three member supervisory committee, a five-member sub-supervisory committee was formed comprising two officials from both the respective States and a member from the Central Water Commission acting as the head.
Tension rose again when Tamil Nadu opened 10 shutters of the dam after midnight without giving proper warning, causing floods along the banks of the Periyar River. It was the 4th time in a month that Tamil Nadu had opened the shutters of the dam after 10 pm. This resulted in the Chief Minister of Kerala, Pinarayi Vijayan sending a letter to his Tamil Nadu counterpart to open the shutters of the dam only at daytime and with proper warnings.
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