Changes in River Water Quality for 30 Years in a Watershed Characterized by Population Decline

A long-term investigation on river water quality (nitrogen, phosphorus, and organic matter) in a watershed characterized by population decline was conducted for 30 years since 1991 with weekly intervals. During the survey period, the population and arable land were uniformly decreasing, with the population decreasing by about 25％ and the arable land by about half, indicating a clear decrease in human activities that cause water pollution. However, the water quality for the entire survey period did not show any visible downward trend; nitrogen increased in the first half of the period and decreased afterward, and the decreasing trend in phosphorus was limited to middle stream point after 2010. A possible factor explaining the change in nitrogen is thought to be the amount of nitrogen deposition from the atmosphere that increased and decreased in the corresponding period. Regarding the phosphorus decrease, it is thought that a dam that started storing water in 2010 and where a water bloom has been observed since 2014, temporarily blocked the phosphorus flow in the river.