The Rohal Reservoir and the massive dam system that fed it represent what is believed to have been the largest manmade reservoir in the Khmer Empire, located within the ancient temple complex of Koh Ker in northern Cambodia. Today the reservoir basin — stretching roughly a mile across — appears as swampy, overgrown lowland, its former grandeur only revealed through modern lidar scanning technology. The dam itself, estimated to have run approximately four miles from Koh Ker north to a jungle river, survives mostly as an overgrown earthen wall, with scattered laterite stone blocks hinting at the engineering ambition of the Khmer builders. Koh Ker served briefly as the capital of the Khmer Empire under King Jayavarman IV in the early 10th century, and this water infrastructure was believed essential to sustaining the city in an otherwise water-scarce environment. Gates visited the site alongside archaeologist Sarah Klassen to investigate whether a catastrophic failure of this dam system may have contributed to Koh Ker's rapid and still somewhat mysterious abandonment.
Koh Ker established as the capital of the Khmer Empire under Jayavarman IV; the Rohal Reservoir and dam system believed constructed to supply the new city with water
The Khmer capital shifted back to Angkor; Koh Ker was largely abandoned within decades of its founding — the dam failure is proposed as a contributing factor
Gates investigates the site with archaeologist Sarah Klassen, examining lidar data that reveals the full scale of the dam and the evidence of its catastrophic spillway collapse (S12E08)
Archaeologist Sarah Klassen, who appears on camera with Gates in S12E08, has been studying Koh Ker using airborne lidar — a remote-sensing technology that strips away jungle canopy to reveal ancient structures beneath. Her work helped establish the true scale of the dam system, which lidar suggests stretched roughly four miles northward to tap a jungle river and channel water into the Rohal and other reservoirs that kept the city alive. Without that water infrastructure, as Klassen explains to Gates, Jayavarman IV's ambitious capital simply could not have functioned.
Mainstream archaeology broadly agrees that Koh Ker was an extraordinary but short-lived experiment in empire-building. Jayavarman IV moved the Khmer capital there around c. 928 CE, constructing an ambitious complex of temples, reservoirs, and infrastructure in what was a relatively remote and resource-limited location. The capital returned to Angkor around c. 944 CE, and Koh Ker was largely depopulated within decades. Scholars have debated the reasons for this rapid abandonment for years, with explanations ranging from political instability to resource exhaustion.
The dam-failure hypothesis represents a genuinely compelling piece of this puzzle, though it remains a hypothesis. The physical evidence — laterite spillway blocks found hundreds of feet downstream from where they should be — is consistent with a sudden, catastrophic breach rather than slow deterioration. If the Rohal and its associated reservoirs drained catastrophically, the city would have lost its primary water supply almost overnight, making continued occupation extremely difficult. Whether the dam failure was a cause of abandonment, a consequence of declining maintenance as the city emptied, or simply one factor among many remains an open and actively debated question in the field.
Gates' episode contributed a vivid, on-the-ground illustration of what lidar has revealed about Koh Ker's hydraulic engineering — the kind of visual storytelling that makes an overgrown jungle wall suddenly legible as a feat of, in Gates' own words, 'empire-making proportions.' The episode stops well short of claiming the dam failure definitively explains Koh Ker's collapse, and that honest ambiguity reflects where the archaeology actually stands.
The Rohal Reservoir is believed to have been the largest manmade reservoir in the entire Khmer Empire at the time of Koh Ker's occupation in the early 10th century CE.
Modern lidar scanning — the same technology used to map Angkor's hidden urban sprawl — revealed that Koh Ker's dam stretched an estimated four miles through the jungle to tap a distant river, a scale invisible to ground-level survey.
Koh Ker served as the Khmer capital for only roughly 16 years, under King Jayavarman IV, before the seat of power returned to Angkor around c. 944 CE — making it one of the most briefly occupied capitals in Khmer history.
Displaced laterite stone blocks found far downstream from the spillway's original location are the primary physical evidence researchers point to when arguing that the dam did not simply decay but failed suddenly and catastrophically.
Koh Ker is generally accessible to visitors as part of Cambodia's Angkor Archaeological Park ticket system, though it lies roughly 120 kilometers northeast of Siem Reap and requires a dedicated day trip or private transport. The jungle landscape around the Rohal basin and dam remnants can be explored on foot, though much of the dam itself is heavily overgrown and unmarked — guided tours familiar with lidar research are recommended for meaningful context. Check current local advisories and road conditions before visiting, as infrastructure in the area can vary seasonally.
Siem Reap, Cambodia — approximately 120 kilometers to the southwest.
The dry season, roughly November through April, offers the most manageable conditions for exploring Koh Ker's jungle sites, with lower humidity and passable roads. Visiting early in the morning helps avoid midday heat and allows for quieter exploration of the more remote dam and reservoir areas.
Guatemala Snake King Archaeological Sites
The Guatemala Snake King sites share a thematic connection as jungle-buried Maya archaeological complexes where lidar technology has similarly transformed scholarly understanding of ancient urban and hydraulic infrastructure.
Cambodia National Museum, Phnom Penh
The Cambodia National Museum in Phnom Penh houses Khmer artifacts and sculptures — including pieces from the Koh Ker complex — providing essential cultural context for anyone investigating the Rohal Reservoir and the broader Koh Ker story.
Sky Caves of Nepal
Like the Rohal dam investigation, the Sky Caves of Nepal represent an episode where Gates consults with archaeologists to investigate an ancient civilization's infrastructure and the mysterious circumstances surrounding its decline.