The catchment basin of the Kashio River is approximately 84 sq. kilometers. Because the Kashio River is a short, meandering stream, with a minimal descent, it has long been notorious for flooding. The predecessor of the Kashio River was the shallow Ofuna Estuary (大船入江), which existed from the end of the last Ice Age to about 300 BC. Subsequently, earthquakes caused crustal uplift, which cut off the estuary from the bay, making it into a floodplain with the Kashio River at its center. As the river often flooded, it formed a lake that left behind a swamp as it drained. The early lake formed in the floodplain of the Kashio River is probably the lake mentioned in the Enoshima Engi, a history of the area.