We examine mantle discontinuities beneath the United States and Gulf of Mexico using multiple ScS reverberations from earthquakes in Central and South America captured by 65 broadband and long-period seismometers across the United States. The depths of discontinuities and the impedance contrasts across them were estimated using a hierarchical waveform inversion and stacking method. The path-averaged depth of the 410-km discontinuity varies moderately across the study area and is particularly shallow (~395 km) beneath the eastern United States. Topography on the 660-km discontinuity is more subdued and is close to the global mean depth. The 520-km discontinuity is seen consistently across the study area, though both the depth and the impedance contrast of the discontinuity vary significantly. Corridors in the eastern United States and Gulf of Mexico have extremely strong 520-km discontinuities relative to the corresponding 410-km and 660-km discontinuities. We attribute the shallow 410-km and strong 520-km discontinuities beneath the eastern United States and Gulf of Mexico to a locally water-rich transition zone.