Its source is in the Polog mountain above the Tolminka valley, an alpine valley below Bogatin in the Triglav National Park. It is a 10.2-km long river flowing through the Soča valley with a large volume of water, making it one of the major rivers in the Tolmin area. It has carved its course towards the Soča through a narrow valley, which sometimes turns into a wild gorge and is at its most spectacular in the section of the famous Tolmin gorges. When it leaves the gorges, it enters the Tolmin basin where it slows down and flows into the Soča at the foot of the Bučenica hill.
The course of the river keeps changing throughout its length. In its upper course, the river is a typical alpine river, while its lower course is calmer but its alpine characteristics still persist. It contains huge supplies of quality drinking water, but is not intended for water supply.
The majority of the river is open to fishing, save for the section of the Tolmin gorges.
The Tolminka is inhabited by all species of trout and Cottus gobio. When it reaches Tolmin, one can also encounter grayling and the occasional Adriatic chub.