The River Sotla
- Spring: southern slopes of the Macelj hills, 640m altitude
- Outfall: opposite the village of Jesenice into the River Sava, 134m above sea level
- Length: 90km
- Average flow: 9.3m3/s (hydrometric station Rakovec I)
- River basin surface area: 581km2, in Slovenia 451km2
The River Sotla has been a border river between the Austrian and Hungarian part of the Habsburg Empire for centuries, and today it demarcates the border between Slovenia and Croatia.
Except in the upper stream, the Sotla takes its course in very well expressed meanders. By the village of Kunšperk, the river cuts a 2km long gorge. It becomes a real lowland river further along. It flows over a wide floodplain through which it used to wind restlessly. Today, the flow has been mainly directed into a levelled artificial riverbed. The most extensive flood area can be found in Jovsi, however it is not the Sotla flooding the area. The wet grasslands are flooded by the groundwater and precipitation, mostly in late autumn or early spring.
Until 1930, a total of 14 mills operated on the entire river both on the Slovenian and the Croatian side.
Beside several fish species, such as the spiny loach, an important species from a nature protection standpoint, (Cobitis elongata, C. elongatoides and Sabanejewia aurata) or the weatherfish (Misgurnus fossilis), the Sotla and its remaining backwaters are also inhabited by the thick shelled river mussel (Unio crassus) and lately also by the otter (Lutra lutra) and the beaver (Castor fiber). Also the wooded plants on the banks are important – not only do they cast shadows on the water, older trees allow for the survival of the hermit beetle (Osmoderma eremita). The European bee-eater (Merops apiaster) initially nested in eroded riverbed walls, however nowadays it nests in a sand extraction site in Bizeljsko due to a lack of such riverbeds. In the summer, backwaters become lively in rich greens due to several species of duckweed (Lemnaceae), the arrowhead (Sagittaria sagittifolia), yellow flag (Iris pseudacorus), and bur-reeds (Sparganium sp.). Sadly, river banks are more and more frequently populated by foreign invasive species, such as the Japanese knotweed (Fallopia japonica), the Jerusalem artichoke (Helianthus tuberosus), and the wild cucumber (Echinocystis lobata). Among the cattail or the reeds is the nesting area of the European tree frog (Hyla arborea), and beside the common frog (Rana temporaria) and agile frog (R. dalmatina) also the moor frog (R. arvalis). This space also represents the hybrid area between the yellow-bellied toad and the fire-bellied toad (Bombina variegata, B. bombina).
Since the Sotla is an explicitly lowland river, it predominantly houses fish from the ray-finned fish order – carp (Cyprinus carpio),European chub (Leuciscus cephalus) and the common barbel (Barbus barbus). From other orders, there are also pike (Esox lucius) and the wels catfish (Silurus glanis).
The Sotla is protected as a natural heritage of national importance (hydrologic, ecosystem, zoological, records No. 4429) for being an environmentally important area and a special preservation area under Natura 2000 (SI3000303)
The Bistrica and its tributaries
- Altitude at its spring – Glija cave: 480m
- Altitude at the confluence with the Sotla – Polje pri Bistrici: 180m
- River basin surface area: 108km2
- Watercourse length: 32km
The River Bistrica is considered one of the most beautiful and best preserved watercourses in eastern Slovenia. Throughout most of its course, it character is that of mid altitudes. A considerable amount of precipitation, gravel areas, alternating deeper and shallower areas, as well as pools are proof that the river has considerable torrential characteristics. The water is relatively pure, flowing energetically over rocks and through pools. Along its banks, the riverside vegetation has been well preserved. Near Zagaj, the Bistrica enters the vast Bistrica-Sotla plain, which extends into the Kumrovec field across the national border.
Between Trebče and Zagaj, there is a picturesque, approximately 3-kilometre-long, gorge, which is too narrow even for a footpath between Reber and Tisovec. The steep, rocky and forested slopes of Bukovica and Orlica rise above 400m on the southern side.
A long time ago, the Bistrica had plenty of mills and sawmills. In Podsreda, the Levstik and Kukovičič mills have been preserved. They contribute to the hydrologic balance of the watercourse and at the same time bear a large cultural and historical importance.
The River Bistrica with its tributaries represents an important habitat for fish, such as the brown trout (Salmo trutta), the European bullhead (Cottus gobio), and also the lamprey. The water is also home to numerous stone crayfish (Austropotamobius torrentium), but also to the larvae of the fire salamander (Salamandra salamandra), dragonflies (Odonata), caddisflies (Trichoptera), stoneflies (Plecoptera), and mayflies (Ephemeroptera). They are hunted by the protected European ground beetle (Carabus variolosus), the grey wagtail (Motacilla cinerea), and the white-throated dipper (Cinclus cinclus). The common kingfisher (Alcedo atthis) preys on larger prey – smaller fish. Some years ago, also the beaver (Castor fiber) returned to the River Bistrica.