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. 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 was used: channels were built on the slopes, then filled with weathered granite earth. When they were washed with water the lighter earth was pulled away leaving concentrated iron ore (up to 80%) that was collected. In the late Edo era about 80% of iron in Japan was produced in Izumo region. The total amount of sediment resulting from this process that was discharged into the river up to 1950s is estimated at about 200,000,000 m³. Moreover, the felling of trees needed for the furnaces operation led to deforestation and erosion and increased the amount of sediment. 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. It is an example of a , which are common in Japan. The accumulation of sediment in lake Shinji stopped the outflow into lake Nakaumi, transforming the former into a freshwater lake. 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 technique. The greatest change came in 1924 with the dredging of Ohashi River that connected again the lakes Shinji and Nakaumi. In addition, the dams built since the 1960s reduced the volume of sediment transported by the river. Ecology Since the 1980s till 2010s there was an improvement in the BOD values of Hii water. Since 2003 it did not exceed the required value of 1 mg/L (except one instance in 2015). From 1980s until the early 2000s the total nitrogen concentration has risen while phosphorus concentration decreased. Since the early 21st century efforts have been undertaken for restoration of wetlands along the river, since these are important for the local ecosystem. Floods The first reports of floods along Hii river are from Yoro period (717-723 AD). According to later reports, significant flooding occurred about every 4 years,