Trebuščica springs under Mrzla rupa and makes a 14 km long way until reaching Idrijca at Dolenja Trebuša. Its first part (5,7 km) is a fish breeding reserve for the genetically pure marble trout. All the rest (8,3 km) is fishing water. The valley is very picturesque with deep pools, cascades and rapids, the gravel deposits only barely enabling wading up its narrow canyon parts. It is populate by all the species of trout, grayling, in the lower part chub and barbell too.