Abstract
We investigated the influence of water level fluctuation on the spatial distribution of deposited sediment and soil environment in the wetland between dam reservoir and its incoming river, called "ecotone" in this study. The target reservoir was Sagae Dam in Yamagata Prefecture, where the range of annual water level fluctuation rises up to 50m and a relatively large area of the wetland is dried out in summer and autumn. In the ecotone we set five lateral transects (Line-1 to -5) and sampled the sediment, interstitial water and surface water in June, August and October in 2009 and October in 2010. When water level was relatively high (June), a clear longitudinal change was not observed regarding size distribution and organic matter content of the surface soil. In contrast, when the ecotone turned into a riverine zone having low water level (August and October), a clear longitudinal shift of sediment and the downstream transects were observed and characterized by fine sediment with rich organic matter at Line-3 to -5. In October such sediment contained reductive interstitial water with the lowest ORP of - 19 to - 35 mV at Line-4. Stepwise multiple regression analysis found that ORP is negatively correlated with fine organic matter content and relative height. The relation of such sediment property to interstitial water quality is possibly determined by the fact that the sediment containing rich fine organic matter has low permeability and high water content. Overall, the results indicates that the fine sediment containing rich fine organic matter maintains the reductive condition in the soil layers deeper than 30 cm even though sediment has been exposed to atmosphere for two months. It implies that such sediment together with anaerobic organic matter decomposition may promote methane production in the ecotone at the upstream end of reservoir.
