The most significant surface water-flows in the area of the Istrian Region are the Mirna, the Raša, the Boljunčica, the Dragonja Rivers, and the underground Pazinčica. In the water supply sense, there is a significant function of the surface accumulations Butoniga and Boljunčica.
The Mirna River is the longest and the richest Istrian river. Its length is 53 km, it springs near Buzet, and it empties into the Adriatic Sea near Novigrad.