The Bača winds through the narrow Baška Grapa valley, from below the mountains of Slatnik and Lajnar towards the Idrijca, all the while accompanied and crossed by the road and railroad. At the village of Bača pri Modreju, the river joins the Idrijca. It covers the distance of 21.4 km.
The first three kilometres of the river are breeding waters for the marble trout, while the rest are fishing waters, which are divided into two sections by the dam in Klavže.
The upper part is inhabited by all species of trout, save the rainbow trout, as well as Cottus gobio, some grayling and Alburnus a. alborell – Barbus meridionalis caninus, common minnow and Leuciscus souffia muticellus – as well as numerous Austropotamobius torrentium. In the lower part of the river, the rainbow trout is also present, and the grayling is more common.