In its lower reaches it provides the boundary to the Fukuoka Prefecture to the west.

The Yamakuni River arises on the slopes of the sacred Mount Hiko on the border of Oita and Fukuoka prefectures, flowing towards the sea through the Yabakei and Nakatsu plains, and empties into the Seto Inland Sea.

It is known as the largest rapid river in Kyushu, and when torrential rain occurs in the upper reaches, a large amount of rain flows down in a short time, which has the characteristic of causing flooding.

Its upper reaches include the scenic Marinkyo Gorge.

[1] Drought is also frequent and was recorded in 9 of the first 31 years after the construction of the Heisei flood barrier.