The effects of fluid shear and sedimentation concentration on the aggregation, and especially disaggregation, of fine-grained sediments in lake waters continues to be an important research area. It has been shown in previous studies that the steady-state median floc size decreases as the shear stress increases and also decreases as the suspended sediment concentration increases. Here, the time rate of change of the particle size distribution as affected by aggregation and disaggregation due to fluid shear and to collisions between particles is considered from a theoretical point of view. Approximate value for the coefficients appearing in the rate equation and their dependence on floc diameter, shear stress, and density were determined. In order to explain the observed decrease in floc size as the sediment concentration increases, the theoretical analysis requires disaggregation due to three-body collisions. The theory does not require disaggregation due to fluid shear. For the present range of parameters, fluid shear seems to have a negligible direct effect on disaggregation, while collisions between particles (possibly due to shear but also due to differential settling and Brownian motion) are the dominant mechanism for disaggregation.