Local current density measurements have been correlated with flooding in a single channel Polymer Electrolyte Membrane (PEM) fuel cell in horizontal and vertical orientations. Water formed at the cathode/membrane interface flows through the largest pores in the GDL when driven by a hydraulic pressure sufficient to overcome the hydrophobic surface energy of the GDL's pores. Liquid water drops grow on the surface of the GDL until gravity or shear forces overcome the interfacial tension between the water in pore and the drop on the surface. After drops detach from the pores they aggregate and form slugs. Local current densities drop when liquid water accumulates in the gas flow channel inhibiting mass transport between the gas flow channel and the membrane/electrode interface. Flooding in PEM fuel cells is gravity dependent and the local current densities depend on dynamics of liquid slugs moving through the flow channels.