[6] Alluvial deposits carried by the river joined the Shimane peninsula to the mainland, which may have been represented in the "Kunibiki-shinwa" myth.

About 9000 BC the sea level began to rise and seawater intruded into the low-lying areas in the east and the west ends, between the hilly peninsula and Chūgoku Mountains.

During the temperature peak of the warm Atlantic period (early Jomon) the sea probably separated the peninsula almost entirely from the mainland.

[7] The final step in this change may have happened as a result of an eruption of Mount Sanbe about 1600 BC and the obstruction of the bay by pyroclastic flow that connected it again to Honshu.

According to the Izumo-fudoki annals, the local deity Yatsukamizu-omitsuno(-no-mikoto) said: "The country Izumo, of the clouds rising, is a land like a pile of narrow cloth.

Using a hoe he pulled pieces of land from Shiragi (eastern Silla), Saki Country and other areas and connected them to Izumo to form the Shimane peninsula.

In another local myth the fight of Susanoo against Yamata no Orochi serpent may represent the flood control efforts of people living along Hii.

[6][19][20][8][21] Till the mid-18th century, after reaching Izumo plain, the Hii turned west and discharged into Taisha Bay of the Sea of Japan.

The sediments accumulated in the plain and after large floods in years 1635 and 1639 the river changed its course and it has discharged since then into lake Shinji.

[6][4][7] In the 17th and 18th centuries, the mountains in the upper Hii basin became the most important source of iron production from ironsand in the tatara furnaces.

For this process a technique called kanna-nagashi [ja] (鉄穴流し) was used: channels were built on the slopes, then filled with weathered granite earth.

Moreover, the felling of trees needed for the furnaces operation led to deforestation and erosion and increased the amount of sediment.

[13][6][16][22][23] The accumulation of sediment in the riverbed increased the flood hazard, causing the locals to construct consequently elevated river banks.

As a result today the river flows higher than the surrounding land, at some points the riverbed is elevated 3–4 m above the nearby plain.

[13][6][24][25] In order to both prevent flooding and expand the agricultural land, in the 17th - 19th centuries the river was artificially diverted every 40–60 years; that was called the kawa-tagae (川違え) technique.

In 1832 Hii was linked to lake Shinji by the Shin River, which was constructed to the south of its former course, but accumulation of sediment led to its closure in 1939.

[6][29][7] In the 1990s works commenced on a vast flood control system, including the construction of a discharge channel connecting Hii and Kando rivers.

[7] In the Middle Ages the river comprised an important transport corridor, through which rice and iron were shipped downstream.

[10][32][33] The Obara dam impounds the 60 million m3 large Sakura-Orochi lake that is used for drinking water supply, irrigation, flood control and recreation.