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An optimized parameterization of the non-dimensional longitudinal dispersion coefficients in Greek rivers, Kxa, is described in terms of parameters derived from cross-section river geometry and longitudinal velocity data. Optimization is accomplished, using various combinations of the selected parameters, either for subsets of Kxa values, according to the cross-section aspect ratio range, or for the entire Kxa population. The use of shape factors associated with the river cross-section transverse profiles of depth-mean longitudinal velocities and the normalized intensities of longitudinal velocity variations from the overall mean cross-sectional velocity improves the parameterization. The statistical behavior of Kxa in terms of skewness, flatness and other shape coefficients is also examined. Kxa histograms show that these coefficients have a log-normal distribution. An empirical expression for Kxa is also proposed that takes into account both contributions from the transverse velocity profile and from the vertical profile of longitudinal velocities, at each cross-section, in shaping the value of dispersion coefficients when the aspect ratio of the cross-section is of O(1) and/or when Kxa is of O(1).