Its source is at the foot of Mrzla Rupa and from there it covers 14 km before entering the Idrijca at the village of Trebuša. The upper stretch of the river – the first 5.7 km – is a reserve and is used as breeding waters for the marble trout. The remaining sections of the river are open to anglers. The valley through which it runs is narrow, but numerous smaller gravel deposits allow crossings near deep pools and past cascades and rapids. It is among the most beautiful rivers managed by RDT.
The Trebuščica is inhabited by all species of trout. Large marble trout often enter it to spawn, as well as grayling and Cottus gobio. The lower stretches of the river are inhabited by Alburnus a. alborell, especially Barbus meridionalis caninus, Leuciscus souffia muticellus and Adriatic chub.