Cohesive suspended particles in a river aggregate into microflocs with average diameters of 10-20 μm, which on their arrival in the estuary aggregate to macroflocs with average diameters of 100-200 μm. We performed a laboratory study on samples from the Seine estuary. The experiments show a more or less systematic behavior of floc size with drivers. Macrofloc size relates to turbulence, η, suspended particulate matter concentration (C), salinity (S), time (t), and mineral and biological composition of primary particles M. Generally, floc size increases with C, S, t, and η. We performed Eulerian field surveys of floc size and the same drivers on various sites in the Gironde estuary. Our field samples show an ambiguous behavior of floc size against salinity and suspended sediment concentration. Floc size does not seem to simply increase with C, S, t, and η, but flocs grow and decay with time. [ABSTRACT FROM AUTHOR]