The existence of stream piracy can be inferred in the Niijuku Pass between the Shiroishi River, a tributary of the Abukuma River, and the Ôtaki River which belongs to the water system of the Mogami River. At the pass, a wind gap has formed. Tributaries of the Ôtaki River are developing on the little fan. A shallow valley and knick point are found at the upper stream of the Ôtaki River. The absence of a flat valley floor along this river which forms the eastern part of the Yonezawa Basin is recognized. These facts can be considered the result of stream piracy.
