During repository building measures a natural river was relocated and the water was directed through a new, artificial river bed lined with a concrete basement and local colluvium. Shortly after, centimetre-thick, beige-colored sinter-crusts were observed in the river bed. An environmental monitoring program launched in order to understand the rapid precipitation process revealed pH values up to 12.9 and high Ca2+ concentrations (196 mg/l) at near-freezing water-temperatures. A first set of crystal aggregates collected was found to disintegrate into a light-colored calcite powder at ambient temperature within few days only. Thus, the metastable precipitates were recovered in original solution and stored in a constantly-cooled refrigerator box. Immediate XRD, FT-IR and ESEM analyses revealed “ikaite” – calcium carbonate hexahydrate – being the rarely documented mineral never reported from such an environmental setting before. Ikaite typically forms in a narrow temperature range (<4-8 °C) from strongly supersaturated solutions, e.g. some specific lake-, sea- and spring-water mixing locations, Arctic- and Antarctic sea-ice. In the present case, enhanced portlandite (Ca[OH]2) dissolution from the concrete river basement by water inflowing during wintertime provided exceptional hydrochemical conditions suitable for rapid ikaite crystallization, e.g. of up to 2 kg d-1 m-2 in a particular stream section. Sampling sites from which ikaite was documented yielded pH >11 probably reflecting some lower limit of ikaite formation. Our findings do not support nucleation inhibitors (aqueous phosphate, magnesium, organic constituents), strongly elevated ionic strength (brines), or water-mixing as a crucial demand for ikaite precipitation. In this “natural laboratory” distinct ikaite vs. calcite spatial distribution encountered along the stream, and thus the spatiotemporal evolution of fluid-solid interaction could be studied. Finally, the prominent and widespread ikaite precipitates vanished rapidly during pronounced springtime air- and water-temperature rise >6 °C superimposed on more gradual hydrochemical changes, e.g. decrease of pH and Ca2+. The latter development highlights the critical temperature restriction of ikaite occurrence and stability. Anthropogenic combined with natural hydrogeochemical processes observed in this study are of interest for both applied and fundamental environmental research.