Liquid water transport and flooding in PEM fuel cells is gravity dependent. Liquid water accumulates at the cathode/membrane interface until a hydraulic pressure sufficient overcomes the hydrophobic surface energy pushes the water through the largest pores of the Gas Diffusion Layer (GDL). Liquid water drops grow on the GDL surface until sufficient force overcomes the interfacial tension between the water in pore and the drop on the surface. When the gas flow channels are horizontal, with the cathode facing down, gravity pulls the water drops from the GDL surface permitting stable fuel cell operation. When the cathode faces up the water drops accumulate on the GDL surface creating a mass transfer resistance that cause the local current density to fluctuate.