Water Quality Improvement by Natural Meandering River Surrounded by Woods in Agricultural Watersheds

抄録

Changes on nitrogen concentration and load of river water during down flow were examined in five rivers flowing through agricultural watersheds in the Tokachi district of eastern Hokkaido, Japan. The upper reach of each watershed is intensively covered by cropland whereas the lower reach is covered by forest. The river form also differs between the upper reaches, which have been artificially straightened by meander cutoff, and the lower reaches, which remain in their natural meandering condition. The lower the reach measured was, the lower the nitrogen concentration was, and this decrease in concentration was even more pronounced in the natural meandering sections than in the straightened sections. The decrease of nitrogen load by natural purification in straightened sections and natural meandering sections was examined using L_self, which was introduced as an index of natural-purification-related nitrogen load decrease. In contrast to the changes in concentration, nitrogen load did not decrease in the natural meandering section.