Bull. Jpn. Soc. Fish. Oceanogr. 79(4), Page 277-286, 2015   Effects of the detention pond on sediment condition in estuary zone on Takahashi River Shuzo Takagi1†, Toshiki Iwamoto1, Youhei Nanba2, Shiho Kobayashi3 and Tateki Fujiwara4 1 Okayama Prefectural Technology Center for Agriculture, Forestry and Fisheries, Research Institute for Fishries Science, Setouchi, Okayama 701-4303, Japan 2 Okayama Federation of Fisheries Cooperative Associations, Minami, Okayama, Okayama 702-8024, Japan 3 Field Science Education and Research Center, Kyoto University, Kyoto 620-0861, Japan 4 IDEA Consultants, Inc., Suminoe, Osaka 559-8519, Japan † E-mail: shiyuuzou_takagi@pref.okayama.lg.jp In order to clarify the influence of the discharge from the detention pond on the estuary zone in Takahashi river, flowing to the Straits of Bisan, we researched the sediment conditions and analyzed the distributions of carbon and nitrogen stable isotope (δ13C and δ15N). In the upper reach of the estuary zone, sediment conditions were good, on the other hand, the lower each became worse rapidly. The concentrations of DIN in the surface water and COD value in the sediment of the detention pond were 144.6±34.6 μM and 58.1±9.1 mg·dg-1, individually. These scores were higher than the mean value of the estuary zone (13.7±5.3 μM and 12.2±9.9 mg·dg-1). The value of the δ13C and δ15N on river zone, sea zone and detention pond zone were -26.6‰·11.1‰, -23.2‰·6.7‰ and -26.7‰·2.3‰, individually. The stable isotope values in the three stations around the detention pond showed similar results to the detention pond. These results suggested that sediment conditions in the estuary zone were aggravated by the mud discharge from the detention pond. Key words: Takahashi river, estuary, detention pond, stable isotope, bottom condition, δ15N, δ13C