From June 2004 to October 2005, particle size distributions (PSD) and the mineralogy of inorganic colloids in Lake Brienz, a glacial flour-rich Swiss lake, and in its tributaries (the Aare and Lütschine rivers), were quantified by single particle counting based on light scattering (0.1–2 μm) combined with transmission electronic microscopy coupled with energy dispersive spectroscopy and selected area electron diffraction. The two lake tributaries differ in terms of the geology of their catchment area and in their hydrology: the River Lütschine's glacial regime is unperturbed, while the River Aare has been significantly modified by hydroelectric installations. The dependence of the colloid concentration numbers on colloid diameters can be described by a power-law distribution in all cases. The power-law slopes in the River Aare were consistently smaller, and the total colloid number concentrations consistently higher, (mean values: −3.89 and 6.2 × 108 mL−1, respectively) than in the River Lütschine (−3.61 and 1.7 × 108 mL−1). The measured PSD in the lake were dependent on season and depth: homogenous in the whole epilimnion in winter and modified by river intrusions in summer. In this season, the presence of a river turbid plume above the thermocline is a significant source of colloids in the epilimnion, where their concentration was high (mean value: 5.1 × 107 mL−1). The same main mineral types were found in the different systems: 2:1 minerals, feldspars, chlorite and quartz; with a few particles of oxides. Differences in their relative concentrations in the rivers, essentially reflecting the geological differences of the respective catchment areas, made it possible to trace the origin of the mineral colloids present in lake surface waters throughout the year. The relationship between colloid size and mineralogy was assessed, showing that different minerals have distinct size distributions. In the rivers, where almost no aggregates were observed, PSD would essentially be the result of the nature (particle formation and mineralogy) of the colloids, whereas in the lake, aggregation and sedimentation processes would also play a role, particularly in summer.