An electrochemical fuel cell stack assembly having a variable active area. The fuel cell stack uses a blocking member to selectively block the flow of reactants through channels of the flow fields of the anode and/or cathode flow paths. Blocking portions of the flow paths allows the fuel cell stack to maintain reactant flow velocities in a desired predetermined range. This enables the control and variation of the active area of the fuel cell, enhancing water management of the fuel cell stack.