The Sutla/Sotla River forms a natural border between Croatia and Slovenia. It is a left tributary of the Sava River. The Sutla/Sotla has a course some 90 km long and a basin of app. 600 km2. In the north, the Sutla/Sotla basin borders with the Dravinja basin, in the west with the Savinja basin, and in the east with the Krapina basin. The basin area is very asymmetrical: its right tributaries on the Slovenian side are very dominant, whereas its left tributaries on the Croatian side are very short with small basins and a torrential character. On the Croatian side of the border, the Sutla/Sotla mostly flows along the foothills, cutting only sporadically into smaller valleys.
In its upper course, the Sutla/Sotla has a steep longitudinal slope and torrential flow, whereas in its middle course the longitudinal slope rapidly declines and transforms into lowland flow with large meanders. The Sutla/Sotla Channel is characterized by thick vegetation (trees and brushwood) in most parts of its flow, which causes the formation of “clogs” that cause local flooding during high water levels in the Sutla/Sotla. Furthermore, frequent erosion of the channel slopes in curve concaves causes significant channel meandering as well as damage to the surrounding land. There are a number of water gauging stations in the Sutla/Sotla basin. Historical records show that the average discharge at the Rakovec gauging station (catchment area of app. 560 km2) for the 1926-2014 period was 8.25 m3/s.
At high water events, the Sutla/Sotla River leaves its channel almost along its entire course, flooding the surrounding agricultural areas both on the Croatian and on the Slovenian side. Certain hydraulic structures have been built over the years in certain regions alongside settlements and roads in order to minimize flood damage. The most significant water regulation and protection structure on the Sutla/Sotla is Vonarje – Sutlansko Lake Dam located upstream of the point where the Mestinjščica flows into the Sutla/Sotla. It was developed in 1980 and was supposed to serve for public water supply, irrigation, and flood protection.