LIBRARY: Introducing the TETHYS4ADRION Pilot Rivers: The Isonzo-Soča River

The Isonzo-Soča River is one of the five pilot rivers being monitored as part of the TETHYS4ADRION project.
It is an Alpine river, with an approximate length of 138 km. Of this, 96 km flows through Slovenia, while the remaining 42 km continue into Italy. It originates in the Julian Alps within Triglav National Park, below the peak of Velika Dnina in Slovenia, and flows into the Gulf of Trieste, eventually emptying into the Adriatic Sea near Monfalcone, Italy. Notable tributaries include the Koritnica, Učja, Tolminka, Idrijca, and Vipava rivers, along with several torrent streams. The most powerful of these is the Boka Waterfall (Slovenia), with a height of 106 meters.

The Isonzo-Soča River is recognized for its exceptional water quality. According to the Slovenian Environment Agency (ARSO), the river is classified as being in very good or good chemical and ecological condition. It supports the endemic marble trout (Salmo marmoratus), a rare and protected species native to the northern Adriatic basin. Amphibians such as newts and frogs, as well as birds including the spoonbill, white stork, and white-tailed eagle, also inhabit its riverbanks. The delta of the river (Foce dell’Isonzo in Italian) in Italy is marshy and surrounded by lagoons. As a habitat for many species of birds and other organisms, it is protected as a nature reserve, along with the last part of its course from Monfalcone onwards, under the name Isola della Cona. 

The Isonzo-Soča is a popular summer destination due to its clear water and striking white gravel beds despite low temperatures limiting its suitability for swimming. Between 2004 and 2010, average water temperatures at key locations ranged from 10.5°C to 15.3°C, with maximums between 13.8 °C and 21.3°C. These persistent low temperatures result primarily from the continuous input of cold karst springs along the river’s course up to Solkan.

The Isonzo-Soča river flows through Friuli-Venezia Giulia region (FVGR) in Italy and through the Goriška region in Slovenia.
FVGR has a population of approximately 1.2 million inhabitants. People over the age of 65 make up 26% of the region’s population, and the average age is 47.8 years. According to the EU’s Regional Innovation Scoreboard 2025, this region is identified as one of the top-performing regions in Italy when it comes to innovations performance. It has been ranked as a Strong innovator-.
The most well-known settlements in FVGR, near which the Isonzo–Soča River flows, are the city of Trieste (the regional capital), Gorizia, and Monfalcone. The city of Trieste has a population of 208,777 inhabitants. It is most famous for its commercial maritime port, which holds the status of a Free Port.

In 2023, the average age of the regional population in Goriška region was 45.6 years; 1.5 years above the national average. The proportion of residents aged 65 and over stood at 24.3%, the second highest in Slovenia. The region maintains a stable socioeconomic environment characterized by high employment, strong educational outcomes, and economic resilience. Around 82.4% of employed individuals work within their region of residence. Key industries include metal processing, electronics, mechanical engineering, and furniture manufacturing. Its proximity to Italy fosters robust cross-border economic cooperation.
The region is also known for its natural beauty, cultural heritage, and wine tourism, particularly in the Vipava Valley and Goriška Brda. The region has numerous historic churches, monasteries, and pilgrimage routes. The Monastery of Kostanjevica above Nova Gorica is especially notable—it is the final resting place of the last members of the French royal family, the Bourbons.

Nova Gorica (Slovenia) and Gorizia (Italy) hold the status of European Capitals of Culture for 2025 and are jointly implementing the cultural and artistic program GO!2025.

Hydropower plants on the Isonzo-Soča River and its tributaries represent a significant part of the historical development of energy production in the Alpine region of western Slovenia. Some of the hydropower plants on the Isonzo-Soča river include: Doblar, Plave and Solkan.

The Isonzo-Soča is a major tourism hotspot in Slovenia, popular for kayaking, rafting, fishing, and hiking.
The most iconic structure spaning over the Isonzo-Soča River is the Solkan Bridge– the world’s largest stone arch railway bridge (85 m span).

The most important potential contributors to pollution, especially of plastic litter in the Goriška region (Slovenia) are: municipal and household waste, seasonal increases in tourism, intensive agricultural practices, industrial activities and illegal waste disposal in remote and low-surveillance areas.

While tourism supports local economies, it also leads to disturbances of sensitive riverine habitats and localized pollution (e.g., litter, release of sunscreen, fuel). Irrigation canals divert water for agriculture, reducing natural discharge and affecting wetlands and associated protected areas. Intensive agriculture, particularly in the Vipava Valley and Goriška Brda, contributes to soil and water contamination through the use of phytopharmaceuticals and plastics as well.

This has been produced with the financial assistance of the European Union. The content of the website is the sole responsibility of TETHYS4ADRION project and
can under no circumstances be regarded as reflecting the position of the European Union and/or IPA ADRION programme authorities.