The main dam is an unusual design: steel-faced rockfill, a type with very few examples elsewhere in the world. Part of the soil on which it rests is laterite - the iron-rich red tropical soil that bakes hard when exposed but softens dangerously when saturated. Seven earthfill saddle dams shore up other parts of the reservoir, where the basin's rim dips low enough to let water escape. The reservoir catches the Laloki River and a handful of its tributaries, filling a natural mountain basin surrounded by peaks. The lake that formed has islands in it - some of them still inhabited, because the villages on those high points stayed above the water when the rest of the valley went under. From the main port on the lake, it's about an hour by bus to Port Moresby.

By one account, six Koiari clans lived on the land the dam flooded: Wanowari, Orari, Monitori, Magibiri, Tuiya, and Bemuri. By another account, the Wakai people had four villages here - the two larger ones called Yoadabu and Gebodabu, the smaller two Araidabu and Sabetana. The Australian authorities who governed the Territory of Papua and New Guinea at the time negotiated with these landowners to build the dam. Construction began in 1958. Five years later the water rose, and a way of life ended. Hunting and gathering became fishing, once tilapia, snakeskin gourami, and silver perch were introduced to the new lake. Giant gourami followed in 1966, with little success. The clans adapted. They have spent the sixty years since trying to get the agreements honored.

On 31 January 1992, the government approved annual rental payments of 130,000 kina until 1995. In 1996, the Sirinumu Development Company was set up through PNG Power to handle payments to the landowners. The landowners do not consider this a success. Demands for compensation stretch back to at least the 1970s, and they continue: a 2019 headline in the Post-Courier read "Dam owners push to 'take back PNG.'" In September 2017, the landowners shut down the dam over compensation disputes, and the lights in the capital flickered. In October 2023, the government tabled a 5 million kina payment to settle outstanding demands. Some landowners have formed an independent company to press for a more formal compensation system, separate from the Sirinumu Development Company they feel has not served them.

Port Moresby gets less rainfall than most of mainland Papua New Guinea, and the city's population keeps growing. Sirinumu is the capital's only supplying dam, though it lies outside city limits. Even with it, water access in Port Moresby is low - and the dam itself frequently sits below capacity. A strong El Niño event in 2014-2015 dropped Lake Sirinumu to just 35 percent of capacity. Water rationing followed in the capital. The math is simple and cruel: the same weather patterns that reduce rainfall over the lake also reduce rainfall over the city, so shortages compound. The Port Moresby Chamber of Commerce and Industry has asserted that any shutdown of the dam violates the Essential Services Act. The landowners answer, in effect, that their essential services have never been delivered.

The lake has become something other than what the engineers built. Kayakers and canoeists paddle across the high water where villages used to stand. Tilapia farms near the shore host visitors who come up from Port Moresby to see. The lake serves as a starting point for hikes to the surrounding waterfalls. Many landowners still farm subsistence crops, because the dam's existence did not make them rich, and they supplement with fishing and small tourism ventures. Homegrown food, fishing, and tourism: that is what the valley's economy has become. The capital below gets its lights and its water from the hills above. The hills get their rain, their fish, and a promise of payments that have always been slow to arrive.