Yakushi the Sarugaishi River flows directly south through the Tsukimoushi section of the city of Tōno.

In Matsuzaki the river makes a slow and gentle bend to the west and flows west-southwest through Ayaori.

In Miyamori the Sarugaishi encounters large hills that force the river into a steep and winding canyon which continues into Hanamaki where the Tase Dam impounds the water.

The river then bends back around to flow west-northwest to Towa then winds in a series of loops generally westward before turning directly north and entering the Kitakami River opposite the mouth of the Segawa and just to the north of the Igirisu Coast of the Kitakami.

The Hayase begins in the region of Sennin Pass and empties into the Sarugaishi in downtown Tōno as the largest tributary.