The special basin shape of Island Copper mine pit-lake and its sharp salinity stratification make it an ideal place to investigate ray waves, or internal wave beams, as the more recent oceanographic literature tends to name the observed phenomenon. A first-mode internal seiche oscillates the deep water vertically over the regular bench structure of the sidewalls at a distinct frequency. Theoretical considerations show the disturbances result in internal wave beams traversing the stratified water body at a known angle. Steep sidewalls prevent the ray waves from being trapped in the beach zone. Instead the reflection at the extremely strong pycnocline creates a vertical interference pattern. Here we numerically derive the vertical length scale of this interference to be 0.43 m. This scale is in full agreement with the repeated turbulence pattern discovered underneath the pycnocline.