The Periyar (, meaning: big river) is the longest river and has the largest discharge potential in the Indian state of Kerala. It is one of the few perennial rivers in the region and provides drinking water for several major towns. The Periyar is vital to the economy of Kerala because it generates a significant proportion of Kerala's electrical power via the Idukki Dam and it flows along a region of industrial and commercial activity. The river also provides water for irrigation, domestic water use, and supports a rich fishery. Due to these reasons, the river has been named the "Lifeline of Kerala". Kochi city, in the vicinity of the river mouth, draws its water supply from Aluva, an upstream site sufficiently free of seawater intrusion. Twenty five percent of Kerala's industries are along the banks of the Periyar. These are mostly crowded within a stretch of in the Eloor-Edayar region (Udhyogamandal), about north of Kochi harbor.
Origin and path The Periyar has a total length of approximately and a catchment area of , of which is in Kerala and is in Tamil Nadu.
Sources The source of the Periyar lies high in the Western Ghats. The state of Kerala asserted during the hearings on the Mullaperiyar Dam issue in the Supreme Court of India that the Periyar originates in Kerala, flows entirely through Kerala and joins the sea in Kerala. This was also admitted by the state of Tamil Nadu in the court. Periyar originates at the southeastern border of Idukki district. The source of the river lies in the remote forests of the Periyar Tiger Reserve. The river originates from Chokkampatti Mala, a peak on the southern boundary of the Periyar Tiger Reserve.
Course In its early course, the river flows in a narrow north sloping valley starting from the Chokkampatti hills and extending north about , with a number of sharp lateral ridges running down to it from east and west. After flowing from its origin and on reaching Mount Plateau at its eastern end, it is joined by the west-flowing Mullayar—an important tributary—at Mullakudy. During this journey the Periyar traverses through the Periyar Tiger Reserve, flowing across or forming the boundaries between various sections of the reserve. Going downstream, it defines a part of the border between Periyar section of the reserve in the east and Sundaramala section in the west and then the borders between Moolavaigai and Thannikudy sections in the east and Ummikuppan, Mlappara and Aruvioda sections in the west before reaching the Periyar Lake.
The Mullaperiyar Dam is constructed at the confluence of the Periyar and Mullayar to create the Periyar Thekkady lake and reservoir, as well as the Periyar National Park. The area belonging to Tamil Nadu in the Periyar basin is located far down the river from the Mullaperiyar Dam site. This area is drained by the tributary Nirar, which is diverted to Tamil Nadu as a part of inter-state Parambikulam-Aliyar Project (PAP) agreement.
From Periyar Thekkady lake and reservoir, some water is diverted eastwards to Tamil Nadu via a tunnel. The diverted water, after power generation, is let into the Suruliar river (a tributary of the Vaigai River) thereby resulting in an inter-basin transfer of water. Downstream of the Mullaperiyar dam, the Periyar continues to flow northwestward for , exiting the Periyar Tiger Reserve and passing through Vandiperiyar, Elappara and Ayyappancoil to the Idukki reservoir formed by the Idukki, Cheruthoni and Kulamavu dams. The Idukki dam is constructed across the Periyar river on the famous Idukki gorge formed by the Kuravan and Kurathi hills.
The main Periyar below the Idukki reservoir flows due north parallel to the western edge of the Idukki plateau and is joined by the Perinjakutty from the east and Muthirapuzha from the north. After the confluence with Muthirapuzha, the river flows northwestwards and enters Ernakulam district at Neriamangalam. It is joined by its major tributary, the Idamalayar, upstream of the Bhoothathankettu barrage. After receiving the waters of the Idamalayar river, the Periyar flows west along the midlands of Ernakulam district.
At Aluva, the river bifurcates into the Marthandavarma and the Mangalapuzha branches. The Mangalapuzha branch joins Chalakudy River and empties into the Lakshadweep Sea at Munambam, and the Marthandavarma branch flows southwards, again dividing into two near Kunjunnikkara island, going through the Udhyogamandal area and finally draining into the Cochin backwater system (part of Vembanad Lake) at Varappuzha. The Vembanad backwaters are connected to the Lakshadweep Sea at Cochin and Kodungallur.
Tributaries Its major tributaries are: Muthirapuzha River Mullayar River Cheruthoni River Perinjankutti River Edamala River
Minor tributaries are: Muthayar, Perunthuraiar, Chinnar, Deviyar, Cheruthony, Kattappanayar
Although the Chalakudy river in strict geological sense is a tributary of the Periyar, for all practical purposes it is treated as a separate river by Government and other agencies. Chalakudy river itself has a catchment area of , with in Kerala and in Tamil Nadu.
Basin
The Periyar basin spreads over an area of , most of it in central Kerala. It lies between latitudes 9°15’30"N and 10°21’00"N and longitudes 76°08’38"E and 77°24’32"E. The river drains parts of Idukki, Ernakulam and Thrissur districts in Kerala and Coimbatore district in Tamil Nadu. The basin has an inverted "L" shape with a maximum width at the intersection. The length-width ratio is 6:1. Most of the streams flow in deep gorges with steep valleys along the highlands of the basin. Coming out of the hills, the Periyar flows west and its basin occupies the northern part of Ernakulam district, covering the region from Neriamangalam through Aluva to Varapuzha and Munambam.
The Periyar is an 8th order stream with considerable development of tributary network. The average drainage density of the basin is 2.46 km/ km2 with a total number of first order streams of 15,773. The total stream length inclusive of all orders in the basin is . The river originates from the high hills of the Western Ghats and the influence of tectonism can be evidenced from the stream pattern and presence of structural valleys. Forests occupy nearly (28%) of the basin area whereas cardamom and settlement with mixed tree crops occupy an area of and respectively. The basin is ecologically sensitive. Nearly 80% of the total area situated in the high ranges are susceptible to erosion and mass movements.
Anamudi, at the highest peak in South India, is the highest point in the basin.
Cardamom Hills
The upper part of the Periyar basin extends across the Cardamom Hills. In the north, these hills extend up to the high Munnar saddle running east to Chokkanad Mala and further on to Kolukkumala on the eastern edge of the Ghats. In the south, the hills reach up to the westerly spur at Chokkampatti Mala. This section of the basin is divided into three parts.
Source – Southern Cardamom Hills The southernmost part of the Cardamom Hills i.e. the source of Periyar River is a narrow valley extending south of Kumily. This is the southern extreme of the Periyar basin. It is less than wide and is about long. The upper catchments of Periyar extend as a wedge deep into the northeastern corner of the neighboring Pandalam Hills. This section of the Cardamom Hills extends deep into but is clearly separated from the Pandalam Hills. The main Western Ghats watershed line forming the eastern border of the Cardamom Hills is highest to the southeast of the Kumily Gap. Here the High Wavies branch off towards north-west into Tamil Nadu, with some peaks reaching elevations of over along the border between Kerala and Tamil Nadu. Further south, the ridge along the watershed line is heavily forested. This part of the Cardamom Hills is called the Sivagiri Range and it extends south up to Chokkampatti Mala which is at the junction of Pamba and Periyar River basins. The southern boundary of the Cardamom Hills runs along a westerly spur hill starting from the Chokkampatti Mala which initially runs north and then turns northwest almost parallel to the Sivagiri Range. This ridge separates the basin of Pamba from Periyar. From its source near Chokkampatti Mala initially Periyar flows north between these two heavily forested ridges. The valley floor is at an elevation of and the ridges on either side often reach . This valley of Periyar at its source and the valley of Kunthi River in the Silent Valley are the only two forested higher valleys in the Western Ghats left intact even now. The Periyar Tiger Reserve is mostly within this southernmost part of the Cardamom Hills.
Cardamom Hills plateau A poorly defined ridge that runs from near Chakkupallam (north of Kumily) on the border between Kerala and Tamil Nadu extending southwest to near Vallakkadavu segregates the upper reaches of Periyar from the rest of the Cardamom Hills. Immediately downstream of Mullaperiyar dam, Periyar cuts through this ridge. North of this ridge, the Cardamom Hills extend up to the high Munnar saddle. Here, east of the river channel, the undulating plateau rises up to the watershed line of the Western Ghats. This section drains into the Periyar through the Perinjankutty Ar and the drainage channel is essentially from south-east to north-west. This portion of the Cardamom Hills is on an average high. The northern edge of the Cardamom Hills bordering the High Range Plateau on the other hand slopes steeply to the south and is drained by Panniyar. The Panniyar joins the Muthirapuzha, which drains the High Range Plateau section of the Periyar Basin in the north. The crestline of the Cardamom Hills in this section along the border with Theni and Kambam, in comparison with the rest of the main watershed line of the Cardamom Hills and the High Ranges, is low, reaching up to only about . East of Chakkupallom, a small part of the adjacent Vaigai basin protrudes from Tamil Nadu into Kerala in this section. This portion of the Periyar basin is drier than all other west sloping parts of the Western Ghats. It has no significant extent of natural forests remaining anywhere. At the turn of the 20th century, it was fully covered by evergreen forests.
Idukki Sub-plateau The portion west of the main Periyar channel which flows from south to north here, is the true Idukki sub-plateau. This is the lowest part of the Cardamom Hills sloping towards the north in which the Idukki reservoir is located. It is separated from the rest of the Cardamom Hills by the deep channel of the Periyar River. The sub-plateau extends north from near Ayyappankoil to near Neriamangalam. Its western border is the clearly demarcated scarp face of the Ghats descending to the plains of Thodupuzha. This high western crestline separates the Periyar catchment from the Manimala, Meenachil and Moovattupuzha drainage basins. It ends in the north along the channel of Periyar from Neriamangalam to Panamkutty. The catchment of Kattappana Ar located beyond the southeastern corner of the Idukki reservoir also falls in this portion. A southwest to northeast ridge called the Chekuthan Mala separates the Idukki Sub-plateau from the Peermade Plateau as well as the main mass of Cardamom Hills. Its eastern boundary is a northwest to southeast ridge separating the Idukki reservoir from the Kallar/Perinjankutty tributary of Periyar which drains the main Cardamom Hills. The Idukki Subplateau is at an average elevation of . This area was heavily forested till the 1950s but now only three small fragments of forest vegetation alone remain. The largest is within the Idukki Wildlife Sanctuary. South of Vagamon, the tributaries of Meenachil, Manimala and Pamba have eroded away extensive areas of the Periyar basin between Ayyappankovil and Vallakkadavu. The Periyar only has a narrow strip of the Cardamom Hill tableland to flow northward here.
High Range Plateau