The is a river on the island of Honshu in Shimane Prefecture and Tottori Prefecture, Japan. With a length of 153 km and catchment of 2540 km2, it is the largest river in the east of Shimane Prefecture. It flows through the cities of Izumo and Matsue and through the lakes Shinji and Nakaumi and discharges into the Sea of Japan.
In the antiquity the river was known as "Izumo-no-okawa" (出雲大川, "The great Izumo river").
The River Hii significantly changed its course and transformed the land several times during last 7 millennia. Alluvial deposits carried by the river joined the Shimane peninsula to the mainland, which may have been represented in the "Kunibiki-shinwa" myth. Since the 17th century it flows into lake Shinji, and since the early 20th century continues to the Sea of Japan. Hii river frequently caused floods in its catchment. On the other hand, it was and currently is an important source of drinking and irrigation water. During the Edo period the upper Hii catchment was the largest iron-producing region in Japan. Nowadays the river is dammed for the production of hydropower. The largest dams are Obara and Hinobori.
Geography
The river's source is located on the slopes of Mount Sentsū, in the town of Okuizumo. The river flows northwards through the Chūgoku Mountains and Yokota Basin (横田盆地). Below Kisuki it joins Mitoya river (三刀屋川). In Izumo city it enters the , where it is connected to by the Hiikawa River discharge channel. Then it turns eastwards and flows through Lake Shinji and then through lake Nakaumi, discharging through the Sakai channel into Miho-wan bay of Sea of Japan. The part of the river that connects two lakes and crosses Matsue city is called (大橋川).
The Hii river is 153 km long and the catchment area is 2540 km2; the population of the catchment is about 500.000. It is 19th longest river in Japan, and 29th largest by its catchment area. It is designated as a Class A river system by the Ministry of Land, Infrastructure, Transport and Tourism (MLIT). The bottom gradient is about 1/160-1/700 in the upper reaches and 1/860-1/1500 in the lower part. The mean annual precipitation is about 2300 mm in the upper reaches and 1700–1845 mm in the lower reaches, most of it occurring in the southwestern part of the basin. The annual discharge in the midstream (Otsu, Izumo city) is 1.4 bln m3. The main tributaries of Hii are Ai, Ohmaki, Kuno, Mitoya and Akagawa. In addition, rivers Iinashi and Hakuta discharge into lake Nakaumi.
In the upper part of the catchment over 80% of the land is covered by forests and around 10% by rice paddies.
History and mythology
During the Last Glacial Maximum, the was fully connected to Honshu. The Old Shinji river flowed in the place of modern lake Shinji and Izumo Plain. 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. At that point the Hii river flowed into the Old Shinji Bay, located at the place of modern lake Shinji and Izumo Plain.
Later the sea level receded again. In addition, sediments from Hii and other rivers accumulated in the bay, cutting it off the sea. The final step in this change may have happened as a result of an eruption of about 1600 BC and the obstruction of the bay by pyroclastic flow that connected it again to Honshu. Afterwards rivers Hii and Kando discharged into the Kandono-mizuumi lagune and deposited their sediment there.
According to some researchers, this change may have been reflected in the Kunibiki-shinwa ("Land-pulling myth"). It tells the story of the expansion of Izumo land by pulling to it pieces of "land in excess" from neighbouring areas. 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. First the land was made small. Therefore, it ought to be sewn larger". 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. The land pulled from Shiragi became Kidzuki Cape, beside which Izumo Taisha shrine is located. Its original name was Kidzuki-oyashiro and it was dedicated to Yatsukamizu-omitsuno. In another local myth the fight of Susanoo against Yamata no Orochi serpent may represent the flood control efforts of people living along Hii.