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River basin description
The Reka River
is the widest known sinking stream of classical Karst and it has been
studied since antiquity. The river sinks in the Skocjan Cave system, which
was proclaimed by UNESCO as World Heritage Site in 1986. Then the
groundwater stream flows to the karst springs of the Timav and drains to the
Adriatic Sea in the Trieste bay. The parts of headwater and the spring are
in Croatia.
The Reka River basin is situated on the Brkini syncline
Eocene flysch rocks surrounded by a large karstic region. The basin has an
area of 442 sq. kilometres down to the sink in the Skocjan Caves. Stream
network is well developed on the impermeable part of the basin. The stream
density is about 1.696 km/km2. The water from the northern
karstic formation belongs to the Danube river basin, while the water from
the southern part drains to the Adriatic Sea. There is an underground
bifurcation from which the surface water flows to the Danube River and the
underground water drainage in the Bistrica spring, a tributary of the river
Reka.
There are town Ilirska Bistrica and a lot of small
villages along the valley.
Hydrological and meteorological monitoring
There are three hydrological gauging stations in the basin. Two of
them, Cerkvenikov mlin and Trnovo, are equipped with a recorder. The others
are staff gauges only and some of them have been abandoned. The hydrological
station Cerkvenikov mlin was established in 1951.
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Hydrological regime
A large fluctuation in discharges
can be observed there. The lowest discharges are in the summer time, when
the natural discharge is below 200 litres per second. The infiltration of
water from stream into the underground begins 600 meters downstream of the
Cerkvenikov mlin. It depends on hydrological and morphological conditions
and changes in time almost after each flood wave. During the low flows
almost all the discharge disappears and the Skocjan Caves are without
water. During 1982 and 1983, even the mean flows of the Reka river
disappeared underground before the river reached the caves, which were
without flowing water for 120 days. After that the riverbed clogged again.
Water quality problems
The chemical and woodwork industry
heavily polluted the river, which was one of most polluted streams in
Slovenia. The main pollutants were the fiberboard factory and the organic
acids factory in Ilirska Bistrica. The BOD5 was between 100 and 200 mg/l during
mean flows in 1969 and 1979. The industrial water treatment was unsuccessful
and the Reka river valley was an unpleasant place with specific smelt caused
by anaerobic processes in the river body specially during the low flows with
only few hundred litters per second of discharge. Pollution prevention work
started in the seventies. A wastewater treatment plant and two reservoirs
for low flow recharge were constructed. In nineties the industry production
decreased and the pollution diminished.
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