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Statistical Analysis of Grain–Size Characteristics of Streambed Sediments in River Catchments of the Lake Tana Basin, Northwest Ethiopia

This paper presents a statistical analysis of grain-size distributions in the samples of bed-material load sediments from the Megech, Reb, Gumara, Gilgel Abbay, and Tana West rivers in the drainage basin area of lake Tana. In this study thirty one streambed sediment samples were collected from these five river’s catchment areas. Grain size analysis was performed and descriptive statistics for the grain-size parameters (mean size, sorting, skewness, and kurtosis) have been computed. Data were analyzed by a range of grain size data analysis methods, such as correlation (Pearson’s correlation), bi-variant scatter plots, box-plots, cumulative probability curves and C-M plots to reveal the effect of controlling variables upon the grain size spatial pattern. Multivariate statistical analysis methods, such as One-way ANOVA, Linear regression analysis and Student’s t-test were also used to discriminate within and between different catchment areas of lake Tana. The statistical analysis indicated that grain-size variations among different river catchments are statistically significant while there is no significant variation within a particular catchment except for the Megech and Gilgel Abbay, and also revealed that there is no statistically significant variation in sediments between two adjacent river catchments of the lake except between Megech and Reb, and Megech and Tana West. These variations may be attributed to various variables like drainage density, lithology, diversity of the sources of the sediments, physiographic setting, lateral distance to lake and transport dynamics. Field evidence from this study suggests that variables controlling within river catchment grain-size variability are strongly site specific.