At its highest point it is called the Sugiyachi Stream (スギヤチ沢, Sugiyachizawa) and flows south until it crosses National Route 397.

The Japanese invaders generally built their forts to the south of east or west flowing streams that empty into the Kitakami.

An opposing Emishi fort called the Tonomi Palisade (鳥海冊, tonomi-saku) was established on the north side of the Isawa at an uncertain date.

The Tonomi Palisade was partly obliterated by the construction of the Tōhoku Expressway but there are still ruins preserved, a graveyard and a beautiful lotus pond on the site.

In the Winter of 1623, a Portuguese priest named Diego de Carvalho and eight Japanese men were arrested on the upper reaches of the Isawa River where they had gone into hiding under the protection of Juan Goto, a local Christian leader.