Biophysical Controlling Factors in Upper Catchments and Braided Rivers in Drylands: The Case of a Marginal Graben of the Ethiopian Rift Valley

Abstract

AbstractThe Raya Graben is a marginal graben of the Ethiopian Rift Valley located in North Ethiopia. The purpose of this study is to investigate the impacts of biophysical factors in the headwater catchments on the morphology of braided rivers in the graben bottom over the period between 1972 and 2010. In this study, 20 representative catchments on the escarpment were selected to analyse the relationship between the morphology (length and area) of the braided rivers in the graben bottom and the biophysical controls in upper catchments (vegetation cover, extremely degraded land, catchment area, compactness, and slope gradient in both the steep upper catchments and the gentle graben bottom). Landsat imagery was used to calculate the normalized difference vegetation index (NDVI), and to map vegetation cover and the total length of the braided rivers. Spot imagery available from Google Earth was used to quantify the total area of the braided rivers in 2005. In regression analyses, both length of braided rivers in the graben bottom (p < 0·0001, R2 = 0·549, n = 100) and area occupied by the braided rivers in the graben bottom (p = 0·0001, R2 = 0·585, n = 20) were positively correlated with upper catchment area. The NDVI in the upper catchments shows an increasing trend (R2 = 0·73, p = 0·067) over the last 40 years. The trend in the NDVI and the length of the braided rivers shows an inverse relationship. Hence, upper catchment area and vegetation cover are important factors of the morphology of braided rivers in drylands. Copyright © 2014 John Wiley & Sons, Ltd.