In the early 1970s, Croatia and Slovenia started developing a joint hydraulic engineering system in the Sutla/Sotla basin that was supposed to have the following purpose:

  • Provision of drinking water and industrial water for part of Hrvatsko zagorje (240 l/s) and the Slovenian part of the Sutla/Sotla basin (160 l/s);
  • Regulation of the high and low water regime on the Sutla/Sotla downstream of the system by retaining high water waves and guaranteeing a minimum discharge of 120 l/s;
  • Irrigation of agricultural areas;
  • Recreational fishing;
  • Tourism and recreation

This joint hydraulic engineering system was designed as a multi-purpose storage reservoir app. 6 km long, surface area of 195 ha, with an earthen dam (bottom dam) and a bottom outlet, and a concrete dam upstream (secondary dam), a storage reservoir, a drainage system, a treatment plant, and tourist facilities. With the construction of Vonarje Dam (earthen dam 12 m high) and Prišlin Dam (concrete dam the purpose of which is to prevent the upper part of the valley from turning into a marsh) in 1980, Vonarje – Sutlansko Lake Dam was created. It was supposed to serve for public water supply, irrigation and flood protection (to retain high Sutla/Sotla water wave and increase flood protection for downstream settlements).

Immediately after the storage reservoir had been filled, the quality of water in the reservoir extremely deteriorated, reducing the possibility to use the water. Due to a high risk for the people and the environment, inadequate management and a lack of better remediation measures, the storage reservoir was drained in 1988. Since then, Sutlansko Lake has served as a retention basin within the flood protection system, with a marsh ecosystem formed at its bottom. The entire area of the retention basin, as well as the entire area of the Sutla/Sotla River channel, has been proclaimed a Natura 2000 area as a conservation site for seven fish species and one shellfish species in accordance with the Habitats Directive.

In the early 2000s, it was proposed to rehabilitate and modernize the regulation structures and to change the purpose of the storage reservoir, which had until then been used only for temporary retention of a high water wave. The proposed change of purpose included the following:

  • Retaining water waves
  • Tourism and recreation
  • Recreational fishing
  • Ecosystem improvement
  • Irrigation