The Sê San cascade is a single machine spread over three hundred kilometres. Thượng Kon Tum, Plei Krông, Ya Ly, Sê San 3, Sê San 3A and Sê San 4 were planned together in 2001 as a staircase, with Sê San 4A added later below them all, and the design logic binds them: the large reservoirs at the top regulate the flow that the plants below depend on, so no station on the river is really independent of the others. Completed, the cascade was projected to deliver something in the order of eight billion kilowatt-hours a year to the national grid. Within that system Sê San 4 is the second largest generator, behind Ya Ly. The river itself begins where the Krông Pô Kô comes in from the right and the Đăk Bla from the left; from there it runs 237 kilometres through Vietnam, draining 11,450 square kilometres of a basin that totals 17,000.

Ground was broken in November 2004. The plant was finished in March 2010 and the works formally signed off in December 2012 — three units, 360 megawatts, sunk into the gorge country where the lower Sê San cuts deep between high ridges. That terrain is the reason the dam is here at all: the upper basin is gentle rolling hill country with modest gradients, while downstream the valley narrows into steep-sided ravines, and steepness is what a turbine wants. The site straddles a provincial line, with Ia Tơi commune on one bank and Ia O commune on the other. That line moved in 2025, when Vietnam's provincial mergers folded Kon Tum into Quảng Ngãi, so the dam now stands with one foot in Quảng Ngãi province and the other in Gia Lai. The concrete did not notice.

A design output figure is a promise about an average year, and the Central Highlands do not reliably supply average years. In 2011, only months after Sê San 4 came fully online, the dry season across the region drew the reservoirs down so far that hydropower stations were reduced to running a few hours a day. The turbines were there; the water was not. This is the structural weakness of a river system with two hard seasons and no middle: the wet months deliver almost everything, the dry months deliver almost nothing, and the storage upstream can only smooth so much. When drought bites, output to the national grid falls sharply and there is no engineering answer to it, only patience.

The answer that did emerge was to stop relying on the water alone. A 49 megawatt-peak solar plant was built on land belonging to the hydropower station on the Gia Lai side, its entire EPC contract financed by the Agence Française de Développement; the credit agreement was signed at AFD headquarters in Paris on 28 October 2019. The pairing is elegant on paper. The months when the reservoir is lowest are precisely the months of hardest, clearest sun, so the panels are strongest when the turbines are weakest, and both feed the same substation and the same transmission line. It is a small installation next to 360 megawatts of hydro. It is also an admission, in steel and silicon, that the river cannot be counted on by itself.

Drive seventy kilometres west from Pleiku and you reach Ia O commune, where the reservoir spreads out behind the dam and a fishing village floats on it. These are households making a living from an impoundment that did not exist a generation ago, on water that covers ground people used to walk. Mo waterfall stands about a kilometre from where the Ia Khai stream once met the Sê San; that confluence lies beneath the reservoir now. Tourism here is new and thin — mostly Vietnamese visitors, few services, boats, cashew groves and rubber on the way in. Downstream the story continues without Vietnamese input: the Tonlé San crosses Ratanakiri and Stung Treng, picks up the Srepok and the Sekong on the way, and Cambodia completed its own Lower Sesan 2 dam in September 2017. One river, two national plans, and a basin whose governance has to cross a line the water ignores.