Sill, River, Tyrol, southern tributary of River Inn, 35 km long, rises east of the Brenner Pass at the Wildseespitze Peak (2,733 m) and flows through the northern (Austrian) part of the Wipptal Valley. The Brennerautobahn (A 13), an important north-south connection, runs through the valley. The Silltal Valley is confined by the Tux Mountains in the east and by the Stubai Alps in the west. Most important tributaries: Navisbach stream (Navis Valley), Gschnitzbach stream (Gschnitz Valley) and Seebach stream from the west. Villages: Vill (817 m), Igls (870 m), Natters (783 m), Mutters (830 m), Matrei am Brenner (990 m), Steinach am Brenner (1049 m) and Gries am Brenner (1,164 m). Power stations: upper and lower Sill power stations and Brenner Pass power station.