Hozu-kyo valley is so narrow that it inhibits the flow of flood water caused by heavy rain and easily inundates Kameoka City and Funai-gun County upstream, acting as a natural flood control basin.
保津峡で川幅が狭くなる為に洪水流下能力が著しく阻害され、上流の亀岡市や船井郡は大雨が降ると容易に湛水し易くさながら天然の遊水地状態になる。