How Athens Gets Its Water: The Mornos–Yliki System Explained

Athens, a city of nearly 3.7 million people, faces a fundamental paradox: it sits on the Mediterranean coast yet depends almost entirely on freshwater reservoirs located more than 200 kilometres away in the mountains of central Greece. The city's sophisticated water supply infrastructure is a testament to Greek engineering but also reveals the deep structural vulnerability of urban water systems in a drying climate.

The story of Athens' water supply is really the story of the Mornos–Yliki aqueduct system, a vast network of tunnels, pipelines, and dams that delivers water from the western Peloponnese and central Greece to the capital. Without it, Athens would run out of water.

The Four Pillars: Mornos, Yliki, Evinos, and Marathon

Athens' water comes from four major reservoirs, each serving a distinct role in the city's supply chain:

Mornos Reservoir is the cornerstone. Completed in 1979 in the Peloponnese, it holds 780 million cubic metres (MCM) of water — more than any other in Greece — and alone supplies roughly 40% of Attica's total water demand. It is the most distant from Athens, located about 210 kilometres away, yet it is also the most vital.

Yliki Reservoir, also in the Peloponnese but closer to Athens, holds 230 MCM and contributes about 30% of supply. It benefits from slightly higher rainfall patterns in its catchment area and serves as a secondary buffer to the system.

Evinos Reservoir, located in the Pindus Mountains in western Greece, holds 200 MCM and accounts for roughly 20% of Attica's water. It feeds into the broader system through a complex series of tunnels and interconnections.

Marathon Reservoir, built in 1929 near the ancient Marathon battlefield, holds only 41 MCM but serves as a critical emergency reserve and backup supply. At 95 years old, it is one of Greece's oldest dams and historically was Athens' sole water source before the western reservoirs were constructed.

These four reservoirs are not randomly distributed. Their locations were chosen by 20th-century hydrologists based on rainfall patterns, geology, and proximity to Athens. The result is a deliberately engineered catchment area spanning thousands of square kilometres of central and western Greece.

The Mornos–Athens Tunnel: One of Europe's Longest Water Aqueducts

What makes this system remarkable is the infrastructure that connects these distant reservoirs to Athens. The crown jewel is the Mornos–Athens tunnel, a 105-kilometre underground aqueduct that is one of the longest water transfer tunnels in Europe.

Completed in 1981, two years after Mornos Reservoir itself was finished, this tunnel is a feat of civil engineering. It is largely gravity-fed, meaning water flows downhill from the reservoir in the Peloponnese all the way to Athens with minimal pumping required. The tunnel burrows beneath mountains, valleys, and towns, delivering up to 650 million cubic metres of water annually during normal years.

The tunnel is not a single continuous bore. Instead, it comprises multiple sections with access points, pressure regulators, and diversions that allow water to be extracted at intermediate locations. Water from Yliki and Evinos feeds into this system at strategic junction points, creating an integrated network where all four reservoirs contribute to a single pipeline feeding Athens.

This design was revolutionary for its time. Rather than building separate aqueducts for each dam, engineers created one massive system that could draw from all four sources flexibly, depending on their fill levels and seasonal demand patterns.

EYDAP: The Utility That Manages It All

The Hellenic Water and Sewerage Company (EYDAP) operates this entire system. EYDAP is responsible for:

Monitoring dam levels and rainfall across the catchment area
Managing the daily flow from all four reservoirs into the tunnel system
Distributing water to 1.3 million households in Athens and surrounding areas
Operating desalination plants that supplement the system
Maintaining the aging aqueduct infrastructure

EYDAP publishes daily data on reservoir levels, which is publicly available online. This transparency allows researchers, journalists, and concerned citizens to track the city's water reserves over time.

System Capacity and Normal Demand

At full capacity, the four reservoirs hold approximately 1,251 MCM of water. In normal years with average rainfall, this is sufficient to meet Athens' demands plus industrial and agricultural allocations across Attica.

The typical consumption pattern is:

Domestic supply (households and businesses): roughly 600 MCM annually
Agricultural irrigation: roughly 300 MCM annually
Industrial use and losses: roughly 150 MCM annually

Total normal demand is approximately 1,050 MCM per year, meaning the system can theoretically sustain supply for roughly 14 months at full capacity without any rainfall. In practice, the system operates more conservatively, with restrictions triggered when combined reservoir levels fall below 40%.

How Water Flows Through the System

The journey of a litre of water from Mornos to an Athenian tap typically works like this:

Water sits in Mornos Reservoir, where it may remain for months or years depending on demand and other sources.
When demand requires it, water is released through outlets at the base of the dam into the 105-km tunnel system.
As it travels through the tunnel, it may be diverted to intermediate communities that tap the aqueduct.
Water from Yliki enters the system at a junction point, increasing flow.
After approximately 48 hours of travel, water reaches the outskirts of Athens in a massive pumping station near the city.
EYDAP's treatment plants chemically process and disinfect the water.
The treated water enters Athens' domestic distribution network, eventually reaching homes across the city.

The entire system relies on gravity for most of its operation, which is why the westerly location of the reservoirs is so important — they are geographically higher than Athens and the surrounding plains.

Climate Vulnerability: The System's Greatest Risk

This sophisticated infrastructure was designed for a climate regime that no longer exists. It assumes a particular pattern of rainfall: wetter in winter and spring, drier in summer and autumn. It also assumes that the system will refill completely most years during the winter wet season.

Climate change is undermining both assumptions. The eastern Mediterranean is warming faster than the global average, and rainfall patterns are becoming more erratic. Winters are bringing less rain, not just in 2025–2026 but year after year. When dry years occur consecutively — as they have in 2024–2026 — the system's reserves dwindle rapidly.

As of early 2026, Mornos Reservoir stands at roughly 45% capacity, down from 85% just 18 months earlier. Similar declines are visible in Yliki and Evinos. Marathon, the backup, is also depleting.

If this pattern continues, Athens will face water restrictions within years rather than decades.

What Comes Next: Desalination and Adaptation

Athens is not entirely dependent on the western reservoirs. The city operates several desalination plants that can convert seawater into fresh water. These plants currently provide roughly 25% of domestic drinking water but can be ramped up to higher capacity if needed.

However, desalination is expensive (both financially and in terms of energy consumption) and cannot scale to meet 100% of demand. It is an insurance policy, not a primary solution.

The Greek government has announced a €2.5 billion water investment plan focused on:

Upgrading existing desalination plants
Building new desalination capacity
Repairing aging aqueduct infrastructure and reducing leakage
Investing in wastewater recycling for irrigation
Supporting agricultural transition away from water-intensive crops in low-rainfall regions

But these measures will take years to implement. In the immediate term, Athens is facing choices: Either increase desalination significantly (at higher cost), restrict agricultural allocations, or impose summer water rationing on households.

Understanding how the Mornos–Yliki system works is essential for understanding Athens' water future. The system is a marvel of 20th-century engineering, but it was built for a climate that is rapidly disappearing. The city's water security now depends on equally ambitious adaptation.