There is an enormous potential for energy generation from the mixing of sea and river water at global estuaries. Here we present a novel approach to convert this source of energy directly into hydrogen and electricity using Reverse electrodialysis (RED). RED relies on converting ionic current to electric current using multiple membranes and redox based electrodes. A thermodynamic model for RED is created to evaluate the electricity and hydrogen which can be extracted from natural mixing processes. With equal volume of high and low concentration solutions (1L), the maximum energy extracted per volume of solution mixed, occurred when the number of membranes is reduced, with the lowest number tested here being 5 membrane pairs. At this operating point, 0.32 kWh/m3 extracted as electrical energy and 0.95 kWh/m3 as hydrogen energy. This corresponded to an electrical energy conversion efficiency of 15%, a hydrogen energy efficiency of 35% and therefore a total mixing energy efficiency of nearly 50%. As the number of membrane pairs increases from 5 to 20, the hydrogen power density decreases from 13.6 W/m2 to 2.4 W/m2 at optimum external load. In contrast, the electrical power density increases from 0.84 W/m2 to 2.2 W/m2. Optimum operation of RED depends significantly on the external load (external device). A small load will increase hydrogen energy while decreasing electrical energy. This trade-off is critical in RED optimization for both hydrogen and electricity generation.