The Syas flows from the Valdai Hills north into Lake Ladoga.

There it accepts the Lunenka from the left, turns northwest and crosses into Volkhovsky District.

The Syas is connected with the Volga through the Tikhvinka, the Tikhvin Water System, and the Chagodoshcha and the Mologa.

The Syas freezes up in November (sometimes in December or even January) and stays under the ice until April.

A fortress of Alaborg was built by the Vikings to guard the approaches to the Syas rapids.