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A planar waveguide laser using a large-core Nd:YAG waveguide and face-pumping by diode bars has produced a multimode power up to 160 W, with direct-contact water-cooling across the largest faces of the waveguide in the lateral direction. A method is reported that measures the nonuniform heat dissipation in the waveguide core and nonuniform water-cooling through deflection of a helium-neon laser beam passing through the waveguide claddings. Temperature gradients resulting from the varying heat transfer coefficient to the coolant are quantified, together with lateral thermal lens effects associated with edge-cooling and nonuniform pumping. The heat flux in the claddings is compared for two different waveguide structures, giving information on multipass face-pumping efficiency and uniformity.