The Bolska is a

river A river is a natural stream of fresh water that flows on land or inside Subterranean river, caves towards another body of water at a lower elevation, such as an ocean, lake, or another river. A river may run dry before reaching the end of ...

Slovenia Slovenia, officially the Republic of Slovenia, is a country in Central Europe. It borders Italy to the west, Austria to the north, Hungary to the northeast, Croatia to the south and southeast, and a short (46.6 km) coastline within the Adriati ...

that flows for 32 km before joining the

Savinja The Savinja () is a river in northeast Slovenia which flows mostly in the Upper and Lower Savinja Valley () and through the cities of Celje and Laško. The Savinja is the main river of the Savinja Alps (Sln. ''Savinjske Alpe''). It flows into ...

as its largest and longest right

tributary A tributary, or an ''affluent'', is a stream or river that flows into a larger stream (''main stem'' or ''"parent"''), river, or a lake. A tributary does not flow directly into a sea or ocean. Tributaries, and the main stem river into which they ...

. Its

watershed Watershed may refer to: Hydrology * Drainage divide, the line that separates neighbouring drainage basins * Drainage basin, an area of land where surface water converges (North American usage) Music * Watershed Music Festival, an annual country ...

covers 190.3 square kilometres of transitional territory in central Slovenia, stretching from the Sava Hills to the Menina and Dobrovlje

karst Karst () is a topography formed from the dissolution of soluble carbonate rocks such as limestone and Dolomite (rock), dolomite. It is characterized by features like poljes above and drainage systems with sinkholes and caves underground. Ther ...

plateaus and extending to the southwestern edge of the Celje Basin. The river rises northwest of

Trojane Trojane (; , ''Leksikon občin kraljestev in dežel zastopanih v državnem zboru,'' vol. 6: ''Kranjsko''. 1906. Vienna: C. Kr. Dvorna in Državna Tiskarna, p. 22.) is a settlement in the Municipality of Lukovica in central Slovenia. It lies in the ...

at an elevation of about 815 metres and descends to its

confluence In geography, a confluence (also ''conflux'') occurs where two or more watercourses join to form a single channel (geography), channel. A confluence can occur in several configurations: at the point where a tributary joins a larger river (main ...

with the Savinja near Prebold at 264 metres. The Bolska and its numerous torrential tributaries are characterised by a

pluvial In geology and climatology, a pluvial is either a modern climate characterized by relatively high precipitation or an interval of time of variable length, decades to thousands of years, during which a climate is characterized by relatively high ...

regime with seasonal flooding, making the watershed particularly prone to catastrophic flood events, as demonstrated by the devastating storm of June 1994.

Geography

The Bolska is the largest and longest right

of the

River in Slovenia's Savinja Valley. Its

covers of transitional territory in central Slovenia. The river rises northwest of

at an elevation of approximately between Reznarca Hill () and Javorje Hill (), and flows into the Savinja River near

Prebold Prebold () is a settlement in central Slovenia. It is the seat of the Municipality of Prebold. It lies on the edge of the lower Savinja Valley at the northern edge of the Sava Hills west of Celje. The area is part of the traditional region of Styr ...

at . The total length of the Bolska is between and , with an average

gradient In vector calculus, the gradient of a scalar-valued differentiable function f of several variables is the vector field (or vector-valued function) \nabla f whose value at a point p gives the direction and the rate of fastest increase. The g ...

of 13.8 to 17.2

‰ The phrase per mille () indicates parts per thousand. The associated symbol is , similar to a per cent sign but with an extra zero in the divisor. Major dictionaries do not agree on the spelling, giving other options of per mil, per mill, pe ...

. The Bolska watershed lies between the central northern edge of the Sava Hills, the southern and eastern slopes of the Menina Plateau and the Dobrovlje

plateau, and the southwestern extension of the Celje Basin. Geologically, it occupies the junction of three major tectonic units: the Sava fold, the young tectonic sink of the Celje Basin, and the Savinja Alps.

Hydrology

The Bolska has a pluvial river regime, with highest water levels in November and December and secondary peaks in March and April. The watershed receives between and of precipitation annually, the wettest months being July and August in summer, and November in autumn. Numerous torrential tributaries feed the river. Its right-bank tributaries, draining the area between the southern divide ridge and valley edge, include the Zaplaninščica, Kučnica and Konjščica; these typically have gradients of 26 to 38‰ and may become destructive torrents during heavy rainfall. Left-bank tributaries such as the Motnišnica and Merinščica drain the karst plateaus and display somewhat different hydrological behaviour.

Floods and environmental impact

The Bolska watershed is exceptionally prone to floods, most frequently in winter. Historic catastrophic floods occurred in January 1799; between September 1842 and January 1843; in 1848; and in September 1926 and 1933. A particularly devastating storm on 28 June 1994 dropped more than of rain in under two hours, causing all tributaries to overflow. The flood destroyed of local roads, rendered impassable and washed away 34 bridges, inundated of farmland, and damaged 176 buildings. About 450

landslide Landslides, also known as landslips, rockslips or rockslides, are several forms of mass wasting that may include a wide range of ground movements, such as rockfalls, mudflows, shallow or deep-seated slope failures and debris flows. Landslides ...

s were triggered, fundamentally altering the landscape. Damage estimates reached 2.2 billion Slovenian

tolar The tolar () or Jáchymovský tolar was a silver coin minted in the Kingdom of Bohemia from 1520 until 1672 in Jáchymov (German: ''Joachimsthal''). The obverse of the coin depicts Saint Joachim with the coat-of-arms of the noble family Schlik ...

s, or 4.6% of the

GDP Gross domestic product (GDP) is a monetary measure of the total market value of all the final goods and services produced and rendered in a specific time period by a country or countries. GDP is often used to measure the economic performance o ...

of the

Žalec Žalec (; ''Leksikon občin kraljestev in dežel zastopanih v državnem zboru,'' vol. 4: ''Štajersko''. 1904. Vienna: C. Kr. Dvorna in Državna Tiskarna, p. 44.) is a town in central Slovenia. It is the seat of the Municipality of Žalec. It lies ...

district.

References

External links

* {{Hydrology of Slovenia Rivers of Styria (Slovenia)