A reverse electrodialysis (RED) system utilizes the transport of cations and anions from controlled mixing of saline water (e.g., seawater) and freshwater (e.g., river water) through selective ion exchange membranes for power generation. Sodium chloride alone has been widely used to create power via salinity gradients in lab-scale RED systems. In an effort to simulate realistic salinity conditions in the natural water environment, in this study a new RED model was developed to quantify the power generation with coexisting monovalent and multivalent salt ions. The effects of different flow rate ratios (saline water flow, ØS, over freshwater flow, ØF) and intermembrane distance ratios on power density (amount of power per unit membrane area) were investigated. Our results indicated that magnesium sulfate, sodium sulfate, and magnesium chloride in the feed solutions of the RED system led to a 9–20% lower power density than when sodium chloride was the single ion source, largely because of the higher internal stack resistance of the multivalent ions. Higher power densities could be achieved with higher flow rates in the saline water compartment and shorter intermembrane distances in the freshwater compartment. For example, the power density increased by approximately 11% when the flow rate ratio was 5 compared with 1; similarly, an intermembrane distance ratio of 8 yielded an approximately 85% increase in power density compared with a ratio of 1. The goal of the present work is to advance our understanding of RED systems working in realistic salinity environments.