The spring of the Idrijca river is in Mrzla Rupa, at the end of the Vojsko plateau (Slov. Vojskarska planota) at an altitude of 960 m. After about 60 km the river flows into the Soča river at the town of Most na Soči. It has quite a few right and left tributaries. Almost the entire river basin of Idrijca and its tributaries upstream from Idrija has been protected as the Zgornja Idrijca Regional Park since 1992 and is part of the extensive Trnovski Gozd – Nanos area, which is one of the Natura 2000 zones. The area is very interesting because of its diverse rock composition and the very uneven undulating relief, almost entirely covered with fir-beech forests, which hide many natural sights (karstic springs, forest reserves). The entire river bed of Idrijca from the spring to the mouth including the tributaries is registered as a valuable natural feature of national importance and at the same time also included in the Natura 2000 areas as a special unit.
Due to its beauty and cleanliness, the Idrijca river attracts many visitors and fans of water sports (canoe, kayak). Idrijca is a true paradise for fans of sports fishing, since it is exceptionally rich in fish. Characteristic of the Idrijca river as a left tributary of the Soča river, is the pure Soča marble trout, whose breeding creek is in the Zakojška Grapa gully. Other types of fish that can be found in Idrijca are: rainbow trout, fish hybrids, grayling, European chub, and brown trout. The temperature and the cleanliness of the river attract many swimmers in summer. The most popular bathing area is near the Kurn’k bar in the village of Reka.