Karst of East Herzegovina: polje, ponor, estavelle and spring in local examples

When guests see a river vanish into the ground or a polje under water in winter, a host needs the right terms and local examples. The facts here come from Hidroelektrane na Trebišnjici (the Trebišnjica hydropower company), the municipalities of Gacko and Berkovići and the Tourist Organisation of Bileća, and the examples come from Gacko, Nevesinje, Bileća, Berkovići and Popovo polje.

In short

• The karst poljes of East Herzegovina step down in a cascade on four height levels, from about 1,100 m above sea level to the sea (HE na Trebišnjici).
• Before the hydropower plants were built, Popovo polje flooded from November to April, forming a lake of up to one billion m³ (HE na Trebišnjici).
• The Dabar hydropower plant, now under construction, carries water from Nevesinjsko polje through a tunnel into Dabarsko polje and on towards Bileća lake. The tunnel was holed through in October 2025, and the plant is announced to start running in 2028 (HE na Trebišnjici, HE Dabar; Trebinje Danas, 2 November 2025).

Terms guests should hear

Polje. A polje has a flat floor and steep sides, and water appears in it and disappears at its edge. The poljes of East Herzegovina lie at very different heights: Gatačko at 920 to 960 m (Gacko LEAP), Plansko near Bileća at 609 m (TO Bileća, Nature), and Dabarsko at 470 to 550 m according to Berkovići municipality (Berkovići municipality).

Ponor. An opening through which water goes underground. The bed of the Trebišnjica is full of ponors: the ponor zone begins upstream of Trebinje, and the number of ponors keeps growing downstream (HE na Trebišnjici).

Estavelle (estavela). An opening that, depending on water levels, sometimes swallows water and sometimes throws it out. The Tourist Organisation of Bileća describes estavelles as reverse ponors that seasonally throw water out instead of swallowing it, and mentions the estavelle "Duvalo" near Grančarevo, which sometimes throws out water even in a summer drought (TO Bileća, Nature).

Spring (vrelo). A place where underground water comes to the surface. The Trebišnjica rises near Bileća, at 330 m above sea level (HE na Trebišnjici), south of the town (TO Bileća, Nature).

Doline (vrtača). A smaller closed hollow in karst. In Gacko the upper polje is a karst basin whose floor is "covered with numerous dolines" (Gacko LEAP).

Sinking river (ponornica). A river that flows underground for part of its course. Before the hydropower plants were built, the Trebišnjica was considered the largest sinking river in Europe (HE na Trebišnjici), and its surface course then varied between 35 and 90 km depending on flow (HE na Trebišnjici).

Where the water goes

The answers come from many years of research and observation of surface and underground flows for the Trebišnjica hydropower system.

• Water from Nevesinjsko polje naturally flows towards the Buna and Bunica and through them into the Neretva (HE na Trebišnjici). The Dabar hydropower plant diverts part of this water towards Dabarsko polje (see below).
• Part of the water of Fatničko polje drains through ponors towards the Bregava, and most of it towards the spring of the Trebišnjica (same source).
• Water from Gatačko and Cerničko polje flows underground towards the Trebišnjica and partly into Fatničko polje, where it sinks again and reappears at the Trebišnjica springs (same source).
• In summer the Trebišnjica used to vanish into ponors near Trebinje and largely reappear at the spring of the Ombla (Rijeka Dubrovačka) (HE na Trebišnjici).
• In Gacko, rain on the roof of the Čemerno weather station drains partly to the Black Sea and partly to the Adriatic (Gacko LEAP).

Hydropower and the karst

The karst guests see today has been changed by the hydropower plants. Through Popovo polje the Trebišnjica now runs in a regulated channel 67.8 km long, 61 km of it lined with concrete (HE na Trebišnjici, HE Trebinje II). According to Hidroelektrane na Trebišnjici, the Grančarevo and Gorica dams cut off the natural movement of fauna along the river and changed the conditions for some fish species, so the company started artificial restocking, with the Mirotinj hatchery and the Jazina fish farm (HE na Trebišnjici, construction phases). The 3,240 m connecting tunnel between Dabarsko and Fatničko polje was cut through in 1985, and the tunnel of about 15.8 km from Fatničko polje to the Bileća reservoir was completed in 2006 (same source).

The newest part of the system is the Dabar hydropower plant. The Pošćenje dam is being built on Nevesinjsko polje, water is brought to Dabarsko polje through a 12.25 km tunnel, and it leaves the plant through a 6,750 m canal across Dabarsko polje into the tunnel towards Fatničko polje and Bileća lake (HE na Trebišnjici, HE Dabar). The tunnel from Nevesinje to Dabarsko polje was holed through in October 2025 (Trebinje Live, 24 October 2025); filling of the reservoir is announced for the end of 2027 and connection of the plant to the grid for the first half of 2028 (Trebinje Danas, 2 November 2025).

Using this in an offer

A short walk "from spring to ponor" is enough to start with: pick a place in your village where at least one of these can be seen, and explain it with the terms above.

Most of Dabarsko polje is under water in late autumn and early spring (Berkovići municipality), and the poljes around Bileća take in water in winter (TO Bileća, Nature). Show a summer guest a photo of the same spot under water. Once the Dabar plant is running, flooding on Nevesinjsko and Dabarsko polje may change, so check such descriptions every season.

If your cheese, meat or vegetables come from the polje, tell guests that too.

Safety

Guests should not go into ponors, shafts or caves on their own. Do not let them climb into openings, and take them into caves only with experienced cavers. For general tour preparation, see the knowledge-hub article on guest safety when hiking, cycling and riding.

Sources

• Trebišnjica hydropower plants: before the system was built
• Trebišnjica hydropower plants: basic development concept
• Trebišnjica hydropower plants: construction phases of the Trebišnjica system
• Trebišnjica hydropower plants: HE Trebinje II
• Trebišnjica hydropower plants: HE Dabar
• Trebinje Live, 24 October 2025: Nevesinje - Dabarsko polje tunnel holed through
• Trebinje Danas, 2 November 2025: HE Dabar timeline
• Gacko municipality: LEAP 2025 to 2030
• Berkovići municipality: Resources
• TO Bileća: Nature