Based on this, it is believed that the lake was formed through a process whereby the southward flow stopped due to topographic subsidence, causing the water level of the lake to fall when flow channels to the Japan Sea were created due to further changes, resulting in the lake disappearing; the flow channels then turned into the present-day Yura-gawa River, the Haze-gawa River and the Maki-gawa River running in the city.
これらから、地形の沈降により南下の流れが止まったことで盆地一帯は湖沼を形成、さらなる変化により日本海側への流路ができると湖沼の水位は下がりやがて消滅、現在の市内を流れる由良川、土師川、牧川などに変化していった、という経緯が考えられている。