Laboratory experiments were conducted in order to investigate the effectiveness of a new scour countermeasure consisting of a horizontal extension plate (EP) downstream of a gated spillway-stilling basin structure. These experiments were performed on a 1:30 scale model of one of the South Florida Water Management District (SFWMD) gated spillway structures, the S65E gated spillway, because it has experienced worrisome downstream scour. Currently, riprap is the only scour countermeasure documented for these structures. EPs of various lengths were installed downstream of the stilling basin, and the equilibrium scour was measured. It was found that a plate length of approximately 1.88 times the exiting flow depth was the shortest length that resulted in significant scour reduction and downstream scour translation. Measured flow patterns reveal that the EP altered the flow patterns by transferring the plunging flow downstream and eliminating the near-bed reattachment and surface recirculation, possibly due to a decreased interaction between the plunging flow and the stilling basin face.