In Herzegovina, a river defies the usual flow direction. Krupa, only nine kilometers long, is a unique feature of Hutovo blato because its water can flow in both directions, depending on the Neretva River’s water level.
The Krupa River is the main watercourse of the Hutovo blato Nature Park, connecting Lake Deran to the Neretva, where it flows into it near Dračevo.
Under normal conditions, water flows through the Krupa from Hutovo blato toward the Neretva. However, when the water level of the Neretva rises significantly, an unusual phenomenon occurs: the river changes its course, and the water flows back toward Lake Deran.
As explained by representatives of the Hutovo blato Nature Park, this is not an optical illusion, but an actual movement of water in a direction opposite to the usual one.
How can a river flow upstream?
The explanation for this natural phenomenon lies in the exceptionally small slope of the riverbed.
According to a professional paper published in e-Proceedings of the Civil Engineering Faculty at the University of Mostar, the fall of the Krupa riverbed amounts to approximately 0.05 per mill, or just five centimeters per kilometer, writes N1.
Due to such a small gradient, the rise in the water level of Neretva can change the water level relationships between the two rivers.
Once Neretva swells sufficiently, its water suppresses Krupa toward Lake Deran, causing the Krupa to start flowing upstream.
This does not mean the water is defying the laws of gravity. It still moves under the influence of the difference in water levels, just in the direction opposite to its normal flow.
The reverse flow can also bring turbid water from the Neretva, whose fine sediment settles in the calmer parts of the wetland and influences the shaping of its environment.
A River without a Classic Source
Another interesting feature of the Krupa is that it does not have a classic river source.
Instead, it is an outlet of Lake Deran; it is the watercourse through which water from the lake flows out toward the Neretva.
Krupa is also part of a complex hydrological system of Hutovo blato, which consists of lakes, canals, underground waters, and numerous karst springs.
It is this interconnectedness of different water systems that enables the phenomenon of bidirectional flow, making the Krupa one of the most interesting rivers in Herzegovina.
Important for One of the Most Valuable Wetland Areas
Hutovo blato is of exceptional ecological importance, especially for numerous bird species that migrate between Europe and Africa.
Some birds stay in the area during migration, others winter there, while certain species permanently inhabit the wetland.
Due to its natural value, Hutovo blato was inscribed on the Ramsar List of Wetlands of International Importance on September 24, 2001.
Changes in the water level and flow of the Krupa play an important role in preserving the entire wetland ecosystem.
More than 6,600 Measurements Reveal Changes in the River
In order to better understand hydrological characteristics of the Krupa, the Hutovo blato Nature Park and the University of Zadar installed an automatic hydro-meteorological station as part of the international SMART-Water project.
The station continuously monitors the water level, the surface flow velocity, water temperature, and meteorological conditions.
During the first 277 days of operation, more than 6,600 measurements were collected, showing distinct seasonal differences.
The data collected from October 2025 to the end of June 2026 showed that the highest water levels were recorded during the winter, peaking in early January, while the lowest levels occurred at the end of June.
The water temperature during the observed period ranged between 5.5 and 28.4 degrees Celsius.
Such measurements enable better tracking of hydrological changes and periods of low water levels, while providing crucial data for the conservation of Hutovo blato.
