A water separator especially suitable for use in a vehicular fuel cell system. The separator comprises a housing defining an inlet, an outlet, and a longitudinal through passage extending therebetween. A central shaft extends along the passage and a plurality of modular baffles are stacked on the shaft. Each baffle includes an annular hub portion received on the shaft and a plurality of circumferentially spaced vanes extending outwardly from the hub portion to block a portion of the cross-sectional area of the passage. An actuator drives one end of the shaft and alternate baffles are keyed to the shaft so that actuation of the actuator relatively rotates the modular baffles to selectively vary the cross-sectional blockage defined by the baffles. The actuator is actuated in response to sensed parameters of the cell such as the humidity of a compressed gas stream delivered to the cell, the mass flow rate of the gas stream, the temperature and pressure within the cell, and the electrical current produced by the cell. With this arrangement and with these parameters, the separator may be instantaneously and continuously adjusted to provide a degree of restriction customized to the amount of water entrained in the gas streams being handled by the separator. In an alternate embodiment, alternate baffle modules stacked on the shaft are relatively rotated to vary the restriction provided by the separator utilizing bimetallic coils between the shaft and the hub of selected modules.