The goal of this thesis work is to investigate the influence of soil water and groundwater input on the strontium (Sr) isotope geochemistry of the suspended (1 kDa – 70 µm) and truly dissolved phase (< 1 kDa) of Fe-rich boreal rivers during spring flood with a special focus on the role of suspended Fe. During spring flood a change towards a more radiogenic 87Sr isotope signature was observed in the truly dissolved and the suspended phase of the Kalix as well as the Råne River. This is interpreted to be due to stronger soil water and minor groundwater input in both rivers during spring flood, since with the soil water a partially groundwater-derived 87Sr enriched Fe-rich suspended phase is released. The Kalix Rivers 87Sr isotopic signature was more radiogenic than that of the Råne River over the entire sampling period, which is suggested to be due to a larger soil water and groundwater component in the Kalix compared to the Råne River. During base flow and up until peak spring flood discharge, the suspended phase in the Kalix and Råne River is significantly more radiogenic than the truly dissolved phase. Since the difference in the 87Sr isotopic signature between the truly dissolved and suspended phase was shown to correlate well with Fe concentrations in the suspended phase, adsorption of groundwater-derived more radiogenic Sr on the Fe-rich suspended phase is suggested to be the cause of the difference in 87Sr isotopic signature between the truly dissolved and suspended phase. In summary a model is suggested: groundwater-derived Fe is precipitated with dissolved organic carbon (DOC) at the anoxic-oxidized interfaces within soils, where with the groundwater more radiogenic 87Sr is supplied and coprecipitated. A more radiogenic 87Sr and Fe-OC-rich suspended phase is formed in the soils and released during spring flood.