Quick answer
Long before the boats, one of the world's largest karst springs poured out where Green Canyon now lies. Here is the honest, sourced story of Dumanlı — and where it went.
Beneath the calm emerald water of Green Canyon lies a lost natural wonder: the Dumanlı spring, once proposed in the scientific literature as the largest karst spring on Earth.
Verified July 2026
Where the water really comes from
Most visitors assume the lake is fed only by the Manavgat River. The deeper truth is geological. This part of the Western Taurus is limestone country, riddled with underground rivers, and one of them surfaced here as the Dumanlı spring. According to the peer-reviewed study by Karanjac and Altuğ in the Journal of Hydrology (1980), Dumanlı alone contributed roughly one third of the Manavgat River's entire annual flow.
How big was it?
The numbers are hard to picture. The same study recorded discharges of about 36 to 100 cubic metres per second, a mean of roughly 50, and around 1.6 billion cubic metres of water a year. Even in the dry season a single orifice pushed out about 35.6 cubic metres per second. On that evidence the authors asked, in the paper's own title, whether Dumanlı was the largest karstic spring in the world. That is not tour-brochure language. That is a scientific journal.
Where did it go?
It is still there. It is simply underwater. When the Oymapınar Dam reservoir filled in the early 1980s, the spring that had fed ancient Side's aqueducts for two thousand years was drowned. Karanjac and Altuğ describe it as submerged beneath roughly 120 metres of water pressure created by the reservoir. So when your boat drifts across that green surface, the old spring is running quietly beneath the hull, feeding the lake it can no longer be seen in.
The aqueduct connection
Dumanlı was not just big; it was useful. Its water was carried to the ancient city of Side through a long aqueduct system. According to the Manavgat Chamber of Commerce (MATSO), around forty arches of that aqueduct still survive near the right side of the dam, with remains standing in several locations. Some of the water that Roman-era Side drank came from the very source now hidden below the boats.
Why this matters for your visit
Knowing this changes how the lake feels. The colour, the depth, the stillness — they are not decoration. They sit on top of a genuine natural marvel that the wider world barely knows about. You do not need to invent superlatives about Green Canyon; the sourced story is quietly astonishing on its own.
If you want the broader picture of how the reservoir was made, read our honest explainer on whether Green Canyon is natural or man-made. And if you are wondering what the trip is actually like above the water, our calm-lake guide explains why this sheltered water behaves so differently from the open sea.
A note on the figures
We only publish what a source supports. The maximum depth of the reservoir is often quoted as 100, 120 or 130 metres on tour sites, and those figures contradict each other, so we do not treat them as fact. The one solid depth anchor is the peer-reviewed line that the old spring now lies under roughly 120 metres of water. When a number cannot be traced to a real source, we leave it out rather than dress it up.
FAQ
Can you still see the Dumanlı spring?
No. The spring was submerged when the Oymapınar reservoir filled in the early 1980s. According to the Journal of Hydrology study, it now lies under roughly 120 metres of water and cannot be seen from the boats, though it still feeds the lake from below.
Was Dumanlı really the largest karst spring in the world?
The 1980 Karanjac and Altuğ paper proposed exactly that question in its title, based on measured discharges of around 36 to 100 cubic metres per second. It was among the strongest candidates ever recorded, which is why the claim appears in a peer-reviewed journal rather than only in marketing.
Did the spring supply ancient Side?
Yes. Its water was carried to ancient Side by a long aqueduct system. The Manavgat Chamber of Commerce reports that around forty arches of that aqueduct still stand near the dam, a visible link between the modern lake and a two-thousand-year-old water supply.
Does the spring still affect the lake today?
Yes, indirectly. The huge karst flow that once surfaced as Dumanlı still enters the system underwater, and the wider Manavgat basin is fed by giant limestone springs. That constant, mineral-rich input is part of why the lake stays full and clear.