Periyar River

The Periyar (Malayalam: [peɾijaːr], 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 5 kilometres (3 mi) in the Eloor-Edayar region (Udhyogamandal), about 10 kilometres (6 mi) north of Kochi harbor.

The Periyar has a total length of approximately 244 kilometres (152 mi) and a catchment area of 5,398 square kilometres (2,084 sq mi), of which 5,284 square kilometres (2,040 sq mi) is in Kerala and 114 square kilometres (44 sq mi) is in Tamil Nadu.

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.

In its early course, the river flows in a narrow north sloping valley starting from the Chokkampatti hills and extending north about 25 kilometres (16 mi), with a number of sharp lateral ridges running down to it from east and west. After flowing 48 kilometres (30 mi) 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 35 kilometres (22 mi), 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, 1.5 kilometres (0.9 mi) 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.