The Stung Sen rises in the hills of Preah Vihear province and runs 520 kilometres southwest across the central plain before emptying into the Tonle Sap system — the largest of the eleven main tributaries feeding the great lake, draining a basin of 16,344 square kilometres that collects around 1,500 millimetres of rain a year. Its name carries an old military echo: steung is simply river in Khmer, while saen descends from the Sanskrit word for soldier. On a map the lower river is almost comically indecisive, doubling back on itself across flat ground so many times that a village 80 kilometres downstream by water may be a fraction of that as the egret flies. The proposed dam site is well upstream of all that, on the upper river southeast of Phnom Rovieng, where the valley is still tight enough to hold a wall.

One name does appear in the file. The reservoir, whatever its final size, would put the settlement of Kompong Putrea under water. How many households that means, how many people, what they farm or fish, where they would go — none of it is in the public record. This is the ordinary shape of infrastructure planning at the frontier of a river basin: the megawatts get a number, the capital cost gets a number, and the community that would cease to have a location is a place-name in a sentence. The people of Kompong Putrea know exactly how many of them there are. The document that proposes to move them does not.

To understand why a mid-sized tributary dam is not a small question, look at what the Tonle Sap does every year. When the monsoon swells the Mekong, the Tonle Sap River is forced to run backwards and the lake inflates from something like 2,500 square kilometres and a metre of depth to more than 16,000 square kilometres and depths of nine to fourteen metres. Fish spawn into the flooded forest; when the water withdraws they come out with it, and Cambodia lands roughly 400,000 tonnes of freshwater fish a year, the bulk of it from this system. Around 1.2 million people live off the lake directly, and the fishery is reckoned to supply a majority of the nation's protein intake and a substantial slice of its GDP. That entire machine runs on the timing and volume of the flood pulse. Dams do not only take water; they change when the water arrives, and the flooded forest keeps a calendar.

For all the attention it has attracted, the Stung Sen Dam has never been close to construction. The Cambodia National Mekong Committee ranks it a "second phase" priority — behind Prek Thnot, Stung Battambang and Stung Chinit, three projects that must come first. Decades after the proposal first surfaced there is still no evidence that ground has been broken. It sits in the long file of Cambodian hydropower schemes that exist as feasibility studies, memoranda and line items: not cancelled, not built, simply parked at a stage where a project can remain indefinitely without anyone having to defend it or abandon it.

The neatest thing about the Stung Sen is that the country has already made up its mind about it — twice, in opposite directions. Downstream, where the river meets the lake, 6,355 hectares carrying the same name form one of the three core areas of the Tonle Sap Biosphere Reserve, nominated to UNESCO in 1997 and established by royal decree in 2001: flooded forest, fish nursery, protected ground. Upstream, the same water is a proposed 40-megawatt reservoir. Both designations are real, both are official, and neither one references the other. Whether the dam is ever built may matter less than the fact that no one has yet done the arithmetic to find out what it would cost — which is, in the end, the most revealing entry in the whole file.