The fate of colloidal particles in Lake Brienz and its two main tributaries, the Aare and Lütschine Rivers, was studied over a period of more than a year. Lake colloid loads from the unmodified Lütschine fluctuated significantly across seasons. The highest loads are in summer due to glacier melting. Colloid loads from the Aare reflected the seasonal water use by upstream hydropower plants. A significant number of mineral particles, even outside the theoretical non-settling range, remain in suspension in the epilimnion of Lake Brienz in summer because of the mineralogical nature and shape of the particles and the high input loads. Only 1730 of the 16,800 tons of colloidal particles that entered the lake during the period from 1 June 2004 – 31 July 2005 were exported through the lake outlet. The remainder was presumably lost in the lake through coagulation-sedimentation processes. Coagulation properties of non-fractionated colloid samples in quiescent conditions were studied in the laboratory both in the absence and presence of different organic compounds representative of natural organic matter. Lake Brienz colloids coagulate slowly, as expected from particles (mainly phyllosilicates) bearing a net negative surface charge. Slightly higher coagulation rates were recorded in March and April during the spring algal bloom, which suggests that colloid coagulation is enhanced by carbohydrates. This effect was confirmed in laboratory experiments. However, the concentration of natural organic matter in Lake Brienz is so low that it is not having any significant effect on the fate of inorganic colloids.