A multistage reverse electrodialysis system was studied at the REDstack research facility (the Afsluitdijk, the Netherlands) for over 30 days to describe the performance of such configuration under natural water conditions. The experiments were done with two 0.22 × 0.22 m2 stacks in series comprising 32 cell pairs (3.1 m2 of membrane area) for stage 1 and 64 cell pairs (6.2 m2 membrane area) for stage 2. The total gross power density at the available salinity gradient was stable at around 0.35 W∙m−2. The total net power density, corrected for the initial pressure drop of the stacks, was 0.25 W∙m−2 at an energy efficiency of 37 %. Throughout the operation, due to increased stack pressure drop, the actual total net power density lowered to 0.1 W∙m−2. A distinct behaviour was found for multivalent ions in each stage. For stage 1, Ca2+ and SO42− were transported from the river water to the seawater side, so-called uphill transport. For stage 2, uphill transport was not found, in line with Donnan potential calculations. Stack autopsy revealed microorganisms with sizes ten times larger than the cartridge filter nominal pore size (5 µm) and biofilm covering part of the spacer open area, both contributing to the increasing pressure drop in the stacks. This study showed that stable gross power densities and high energy efficiencies were obtained from feeding natural waters to a multistage reverse electrodialysis system, independent of fouling. In addition, it emphasized the importance of maintaining pumping power losses low for a viable deployment of the technology.