Thenmala Dam

The story of Thenmala Dam

The dam at Thenmala impounds the Kallada where it comes down out of the Agasthyamalai hills, and the sheet of water behind it is what you are actually looking at from every viewpoint, walkway and boat in the eco-tourism area. The dam came first; the tourism was built around the reservoir afterwards, which is the reverse of how most people assume it happened.

It is not, strictly, a dam across the Kallada. Three rivers — the Shendurney, the Kulathupuzha and the Kazhuthurutty — meet inside the impoundment, and the Forest Department's own description is of a dam built across all three. The Kallada as a named river effectively begins at this wall.

As a structure it rewards understanding rather than admiration. It is a straight gravity masonry dam, 335 m long and 85.35 m high above its deepest foundation, built in twelve blocks numbered left to right, with three gated ogee spillways of 12.19 by 9.14 metres between two non-overflow sections. Those first two numbers are the only ones every official source agrees on.

The engineering here is usually written off as having no elegance and needing none, on the grounds that the job is storage and irrigation. Power goes unmentioned. There is a 15 MW hydroelectric station standing at the toe of this wall, two 7.5 MW vertical Kaplan machines, and it is visible in the corner of almost every downstream photograph of the dam with KERALA STATE ELECTRICITY BOARD LTD, KALLADA HYDRO ELECTRIC PROJECT painted on it. The Irrigation Department's own mission statement for the project says water in the canals for irrigation and power generation. We were the only ones who missed it.

The reservoir is the good part. It runs back into forested hills with no development on the far shore, because the far shore is Shendurney Wildlife Sanctuary and cannot be developed. That is why the view holds up: you are looking at a managed water body with a protected catchment behind it, and the absence of buildings is a legal fact rather than an accident.

One practical thing that no listing states plainly: the boating is not at the dam. It runs on the same reservoir at Pallamvetty, beside the project's earthfill saddle dam, about 2.5 km from the main wall — a different place, reached separately, with a battery shuttle to the boarding point. Anyone booking boating at Thenmala Dam is boating nowhere near the structure in the photographs.

The other thing to know before you drive is that this lake has a season of its own. It fills after the southwest monsoon and is drawn down hard through the hot months, because the level is set by irrigation releases rather than by weather. In late April 2026 it stood at 34% of capacity with wide bands of exposed red bank. A March-to-May visitor sees a very different lake from the brochure.

The best hour is early, before the day's visitors arrive and while there is still mist sitting on the water against the forest. By late morning the light is hard and the reservoir looks like what it is — a large flat body of water — rather than the thing that is worth the drive.

Legend & myth

This is the most thoroughly documented dam in Kollam district, and almost every published number about it — including the Central Water Commission's — is contradicted by another official number about it.

Start with the name. Its name is often given as the Parappar dam, with Thenmala merely the place. That was half wrong, and we are correcting it. Every primary source calls it Kallada. The 1984 Kerala government notification that created Shendurney sanctuary says Kallada Dam and Kallada Reservoir. The Central Water Commission registers it as Kallada (Parappar), dam code KL07HH0037. The World Bank's 1982 appraisal calls it the Kallada dam. KSEB calls the water the Kallada Reservoir. The Irrigation Department declines to name it at all and describes a gravity masonry dam across the Kallada river at Parappar near Thenmala. So Parappar is the site exactly as Thenmala is the town — and the name the government files it under is the one nobody uses.

Then the register itself. The Central Water Commission's National Register of Large Dams puts this dam in Pathanamthitta district. It is in Kollam, Punalur taluk — KSEB says so, the Irrigation Department says so, the 1984 notification says so, and the coordinates in the register's own adjoining column say so. The same row classifies it as an earthfill embankment. It is not: the Irrigation Department calls it a straight gravity masonry dam, the World Bank's appraisal calls it a gravity masonry dam twice, and the downstream face in every photograph is stepped masonry with three spillway chutes. The CWC approved the design of a masonry gravity dam and then registered it as an embankment. There is an earthfill element here — a 225.7 m saddle dam at Pallamvetty, 12.51 m at its highest — but that is not the 85.35 m structure the register is describing.

Nobody agrees how much water is behind it either. The World Bank's design figure for live storage was 488 million cubic metres; the CWC says effective storage 507 million and gross 524 million; KSEB says 472. The water spread is worse: KSEB gives 13.72 square kilometres, the CWC gives 23, and the Forest Department's own sanctuary literature gives 26. The highest is nearly double the lowest, which is not a rounding difference. The catchment is the one thing everyone agrees on, at about 549 square kilometres.

And here is the fact that cuts in Thenmala's favour, which we had not previously reported. The CWC's register lists a purpose code for every large dam in India, and the code for this one is I/H/S/T — irrigation, hydropower, water supply and, formally, tourism. Only five of Kerala's sixty-one registered large dams carry that T, and at 85.35 m this one is more than twice the height of the other four. India's first planned eco-tourism destination is not purely marketing: tourism is a statutory purpose of this dam in the Government of India's own register.

One more, from the department that owns it. Nearly every tourism page calls the Kallada scheme the second largest irrigation project in Kerala. The Kerala Irrigation Department's own page opens by calling it the largest. The related claim that this is the longest reservoir in the state is repeated everywhere and sourced nowhere, so we do not repeat it.

The downstream consequence is documented and is almost never mentioned here. The Kallada's floods used to carry sediment to the low islets at the river's mouth, at Munroe Island, and rebuild them each year against the tide. The Kallada Irrigation Project and this reservoir cut that supply. Munroe Island has been losing land ever since — measured at about 14% of Munroe Thuruthu, with Cheriyakadavu at 46.95% — and more than 500 households have been forced to vacate their homes. The 2023 NCESS paper that measured it names this reservoir, together with uncontrolled sand mining, as the key factor.

None of that means the dam should not have been built. It irrigates a very large agricultural area, supplies water and generates power, and those were and are real public goods. What it means is that a page describing this reservoir as simply beautiful, in a district that also contains Munroe Island, is telling you half of something. Both halves are true and both belong here.

The honest bottom line for a visitor: the dam is 335 metres long and 85.35 metres high, and those two figures are stable across the Irrigation Department, the Central Water Commission and the World Bank. Everything else you will read about it — its name, its structural type, the district it stands in, how much water it holds — depends on which government file you open.