The Kolpa is a river that forms a natural border between southeast Slovenia and northwest Croatia. One of Slovenia's best-preserved rivers, it is popular with bathers and fishermen, but urbanisation, tourism and climate change have taken their toll.

Periodic ecological status assessments performed by the Environment Agency show the state of the Kolpa is deteriorating.

In 2014-2019, the upstream section between the settlements Osilnica and Petrina was assessed as very good, while the other two sections lower downstream, the first between Petrina and Primostek and the second between Primostek and Kamanje, were assessed as good.

The monitoring performed in 2020 and 2021 showed the status of the first two sections remained unchanged, while the status of the third section deteriorated slightly and was assessed as moderate.

"This was due to the excessive effect of nutrients on the algae living in contact with the river bed and the aquatic plants," the Environment Agency says.

However, a one-year monitoring can only hint at a deterioration. A more thorough assessment will be carried out after monitoring spanning several years.

Less water makes river more vulnerable

The agency takes samples in Osilnica, Radenci, and Radoviči a few times a year. The values they measure are mostly still within limits, according to the Novo Mesto department of the Nature Conservation Institute.

The Kolpa remains one of the best-preserved rivers in Slovenia, they say. However, increased pollution and lower discharge values have undermined the river's self-cleaning ability.

Analyses of discharge in Radenci - a measurement showing how much water is flowing in a river per a unit of time - show the average and lowest annual values are decreasing. The most intense decrease took place after 1980.

The average discharge fell from 55 cubic metres per second in 1947 to 45 m3/s in 2013, Barbara Kink and Matej Simčič from the Novo Mesto department of the Nature Conservation Institute explained for the Slovenian Press Agency.

Bathing waters still good or excellent

The monitoring and evaluation of bathing waters at the end of the 2021 bathing season showed that the water quality in Primostek, one of the main bathing areas on the Kolpa, did not comply with the bathing water standards as a trend of microbe increase was detected.

"The data tells us the condition is gradually deteriorating, which is why we need to undertake measures before it is too late," Kink and Simčič said.

Despite the deterioration, the Kolpa is still suitable for bathing, with nine areas being defined as bathing areas, including Primostek.

"In the 2023 bathing season, the water in the Kolpa was suitable for bathing in all bathing areas. Its bathing waters were classified as either excellent or good," the Environment Agency said.

Bathing water quality is classified into four categories: excellent, good, adequate and bad. The water must have at least adequate status to be suitable for bathing.

Call for action

Pointing to the trends shown by their measurements, experts warn that action is needed to tackle waste water treatment and come to an agreement about a comprehensive approach to river management at the local, national and cross-national levels.

As the main threats they list waste water discharge as a result of too liberal construction and urbanisation, excessive sporting activities and tourism, unregulated summer traffic and locally intense farming, especially when combined with unregulated waste water treatment.

Agglomerations not being connected to local utility infrastructure, waste water and dispersed settlements were identified as potential sources of pollution in Primostek in a 2022 bathing water quality management action plan.

The Kolpa is a border river, and polluters need to be identified in cooperation with Croatia.

The challenges of comprehensive management, mass tourism, and carrying capacity of the environment need to be addressed.

"As a society we need to realise that rivers and streams are not just water drainage conduits or tourism potential. They are living ecosystems that need to withstand the natural and anthropogenic pressures all year around," Kink and Simčič warn.

Fish population still diverse

There are 33 fish species living in the Kolpa, Danilo Puklavec from the Fisheries Research Institute says. This means the fish population is quite diverse.

In the 35 years since they have been monitoring the Kolpa, no fish species has gone extinct. However, the total catch data in 1995 showed a rather significant decrease of all fish populations, even up to a half or more in some species.

One of the possible reasons for the sudden decrease in fish population between 1990 and 1995 could be that this was when winter snowpack started to decline, which led to smaller amounts of water in the river during the summer, says Puklavec.

Since 2005, when the institute started carrying out intense research of the Kolpa itself, no serious decrease in fish population has been noticed. "The quantity is more or less the same," says Puklavec. He assesses the Kolpa as a clean river.