The Enns forms the delicate backbone of the Gesäuse National Park. It rises in the province of Salzburg (Radstätter Tauern at the foot of the Kraxenkogel, 1,735 m) and collects water along a length of 254 km in a catchment area of 6.080 km². Between its source and its estuary on the Danube River, the Enns transcends a drop in altitude of 1,497 m. This makes it the longest river in Austria both to originate and end within national territory.
The Johnsbach is the most important tributary of the Enns in the National Park. It rises at an altitude of around 1,500 m in the Eisenerz Alps (greywacke zone, above the Grössingeralm mountain pasture). It flows westwards through the village of Johnsbach where it abruptly changes direction and turns 90° to the north, breaking through the Triassic limestones and Dolomites of the Gesäuse mountains. This 8 kilometre V-shaped valley is also known as the Zwischenmäuer ("between walls").
To date we have recorded around 900 points of water (650 springs, 30 sinkholes and ponors, and 115 ponds) in an area of 12,000 hectares. Most of the largest springs emerge from the Dachstein limestone. We find such formations above all in the central and eastern Buchstein and Hochtor range and on the Reichenstein and Lugauer mountains. However, the Gesäuse is also home to "non-karst" springs.
The Sulzkarsee is the only lake in the National Park. The only other standing bodies of water are occasional pools, particularly on the alpine pastures which have been established in areas with sufficient water supply. The Sulzkarsee is subject to strong fluctuations in water level. It is partly fed by underground springs and partly from surface runoff following strong rainfall. The lake drains through a moraine dam.
The Enns and the Johnsbach are the two most notable water bodies in the Gesäuse National Park. The River Enns, which elsewhere is very strictly controlled, flows through the national park for several kilometres in a bed with scarcely any human intervention. The Johnsbach, which flows into the Enns near the willow dome, is still an untamed mountain stream. The habitat along the rivers Enns and Johnsbach in the Gesäuse provides a major contrast to that in the mountainous regions and harbours a unique and varied fauna.