a river in Novgorod and Leningrad oblasts, RSFSR. The Sias’ measures 260 km in length and drains an area of 7,330 sq km. It originates in a lake on Tikhvin Ridge and flows over the Lake Ladoga depression. It empties into the Volkhov Guba at Lake Lagoda. The Sias’ is fed predominantly by snow. The mean flow rate 27 km from the mouth is 53 cu m per sec. The river freezes in November, sometimes in December or January; the ice breaks up in April. The river is part of the Tikhvin water system. The settlement of Sias’stroi is on the Sias’.