It generates a significant proportion of Kerala's electrical power via the Idukki Dam and flows along a region of industrial and commercial activity.

[7] Kochi city, in the vicinity of the river mouth, draws its water supply from Aluva, an upstream site sufficiently free of seawater intrusion.

[28] 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.

[36] 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.

[43] 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[39] to Varapuzha and Munambam.

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.

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.

[47] The southernmost part of the Cardamom Hills i.e. the source of Periyar River is a narrow valley extending south of Kumily.

The main Western Ghats watershed line forming the eastern border of the Cardamom Hills is highest to the southeast of the Kumily Gap.

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.

[22] 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.

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.

East of Chakkupallom, a small part of the adjacent Vaigai basin protrudes from Tamil Nadu into Kerala in this section.

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.

[22] South of Vagamon, the tributaries of Meenachil, Manimala and Pamba have eroded away extensive areas of the Periyar basin between Ayyappankovil and Vallakkadavu.

All the northern slopes of the main Anamalai ridge falling within Kerala drain into Idamalayar and Pooyamkutty Ar, both tributaries of Periyar.

Separating these valleys are a jumble of steep hills and flat topped plateaux with scarp faces rearing up to elevations of around 900 to 1,500 metres (3,000 to 4,900 ft).

The river bifurcates near Aluva into two major distributaries; one continues to flow through Desam as Mangalappuzha and finally drains into the Lakshadweep Sea.

The Chalakudy river also joins to this branch of the Periyar 10 kilometres (6 mi) south-east of Kodungalloor which expands to its maximum width at Munambam.

The scheme consists of the Periyar Barrage at Bhoothathankettu with a catchment area of 3,048 square kilometres (1,177 sq mi) along with a network of canals.

Salinity intrusion and pollution dispersion problems arose in the lower reaches of the Periyar due to non-availability of sufficient quantity of water for flushing.

[56] The Parambikulam Aliyar Project (PAP) agreement was conceived to harness and share the waters in the inter-State rivers in Bharathappuzha, Chalakkudypuzha and Periyar basins.

This entire stretch contains about 600 to 700 square kilometres (230 to 270 sq mi) of undisturbed wet evergreen forests typical of the Western Ghats.

On the other hand, the landscape has also witnessed a variety of developmental activities including commercial plantations, hydroelectric and irrigation projects and other forms of infrastructural development that impede the movement of elephants.

Kalady started featuring in the spiritual map of India only after its rediscovery in the late 19th century by the then Shankaracharya of Sringeri and the subsequent consecration of a temple for Sri Adi Shankara in 1910.

Greenpeace India describes the lower Periyar as "a cesspool of toxins, which have alarming levels of deadly poisons like DDT, endosulfan, hexa and trivalent chromium, lead, cyanide, BHC".

[69][70] Several studies have pointed out that the riverbed has deposits of heavy metals like lead, cadmium, mercury, chromium, nickel, cobalt and zinc and the ecosystem of the river has many dead zones.

[74] The Aluva Environmental Protection Forum[75] started by Sitaraman and others have played a pivotal role in the prevention of illegal sand mining and water pollution in the lower reaches of the river.

The control and safety of the Mullaperiyar dam on the river and the validity and fairness of the corresponding lease agreement have been points of dispute between Kerala and Tamil Nadu states.