The particle size distribution in the Rhine River near Basle (Switzerland) was measured for a period of over 1 year. Peaks in the measured size distributions were consistently observed in the ranges of 100–200, 300–700 nm and 1–3 μm. The size distributions observed did not vary greatly with time or flow rate. Predictions of a classical coagulation/sedimentation model agreed well with these field observations. This agreement between model predictions and field measurements indicated that the unknown (and probably dynamic) “initial” particle distribution may have been quickly transformed, either in the river or in the interstitial soil solution, through coagulation and sedimentation into the characteristic and relatively stable shape experimentally observed.