The river basin has an area of 598 square kilometres (231 sq mi). The major tributaries are the Belca, Zala, Cerknica, and Bača from the right and the Nikomlja, Kanošica, and Trebušica from the left. One of the right tributaries is also the Jezernica River, which originates from the Wild Lake (Slovene: Divje jezero). Being only 55 m long, the Jezernica is the shortest river in Slovenia.
The river has many fish, among which the Salmo marmoratus, the Rainbow Trout, and the Grayling are noteworthy. In the past, timber was driven down the Idrijca to Idrija to be used as pillars in the Idrija mercury mine. Special logging sluices (Sln. klavže) were employed for this purpose from the 17th until the 19th century.