An analysis of previous experimental determinations of aggregate settling rates for particles <100 μm in diameter shows that these particles fall significantly more quickly than Stokes' law predicts, with the deviations being most pronounced for smaller particles. This implies that previous sediment flux calculations which have used a form of Stokes' law to obtain aggregate velocities may have significantly underestimated the contribution from the smaller aggregates. The measured fall velocity distributions are described as the sum of normally distributed populations (four for the lacustrine data, two for the marine) whose means, standard deviations, and weights have been determined. Sample calculations of collision frequency show that because of the increased settling velocities and the use of a distribution of velocities for a given particle size, collisions due to differential settling rates are the governing nonbiological process in the formation of natural aggregates.