Role of wetlands for sediment absorption

Sedimentation from agricultural fields and seasonal euthrophication from non-point source nutrients are the main problems in the Lake Tana, upper Blue Nile Basin (BNB) of Ethiopia. The main objective of this study was to show the role of wetlands on sediment absorption. Two wetlands were selected purposefully to quantify how much sediment and nutrient trapped by the wetlands. All sampled runoff were filtered and measured to determine sediment amount in gram per liter. Only 10 representative samples were subjected to nutrient analysis. Landsat images of 1985 TM, 1995 TM, 2000 ETM+ and 2011 ETM+ were used and supervised classification was applied to know changes of wetlands. In general, about to 86 % of the incoming sediment was absorbed in site 2, whereas 80 % in site 1. Regarding the nutrient loss, 53 and 33 % nitrogen and 50 and 29 % phosphorus absorbed by the wetlands from incoming load in site 2 and site 1, respectively. About 49 % of OM was absorbed by the wetland in site 2, whereas, it was 31 % in site 1. In the three periods, 1985-2011, wetland change was obtained. Nearly 119 square km (27 %) of the wetland was converted to the other land use types in the last 26 years.