Influence of deposition in dam reservoir on the deep marine hemipelagic environment off Niigata, central Japan

It has been well known that dam construction on rivers can cause a decrease in sediment supply and serious coastal erosion, while influence of dams on the sedimentary environment beyond the coastal area has not been well known. The mass accumulation rate (MAR) in the hemipelagic environment (500–700 m water depth) off Niigata, central Japan since the early half of the twentieth century was obtained from core samples, based on excess ²¹⁰Pb activity. Decreases in MAR during the mid-twentieth century were recognized in several core samples located northern off the Agano and Shinano river mouths. Using the CIC model, combining correction for the influence of surface bioturbation based on the ratio of MARs, decreases in MAR were estimated as occurring during 1959–1964 and 1949–1955. These dates correspond with construction of the Okutadami dam (1956–1961), possessing the largest capacity and reservoir sediment volume of 3.9 × 10⁷ m³ in the Agano and Shinano river systems. The other core located NW off the Agano river mouth showed constant MAR through the mid-twentieth century. A synchronicity of huge dam construction and decrease in MAR in deep marine environment, the similarity between silt–clay-sized grain size of dam reservoir sediment and that seen in the hemipelagic environment, and horizontal distribution of decreasing MAR in cores according to north–northeastward sediment transport on shelf, makes it reasonable to interpret that the decrease in MAR in hemipelagic environment was caused by entrapment of silt–clay grains in the huge dam reservoir.