Fractal theory is associated with river dynamics to study the method of quantifying riverbed surface form and its physical significance. Typical bifurcate reaches with severe riverbed erosion and deposition in downstream Three Gorges Dam are taken as the example to analyze the BSD (bed surface fractal dimension) variation and its relation with the erosion-deposition adjustment of the corresponding surface, longitudinal section, and cross-section of each reach. BSD can be used to reflect the nonlinear feature of the bed form variation across time and space, which is different for different reaches on the one hand, and changes along with the riverbed erosion-deposition adjustment on the other. It can also be used to describe the extent of the bed form adjustment as BSD value is positively correlated with the complexity of typical cross-section bed form. As a method of quantifying riverbed surface adjustment, BSD will be valuable for the quantitative analysis of river regime, the discrimination of river pattern, and the prediction of bed resistance tendency.