Large eddy simulations (LES) are performed on a cold rectangular supersonic multistream jet with an aft-deck to understand the interaction of multiple streams in the presence of the deck boundary layer. The primary (core) stream has a single expansion ramp on the upper side. Between the core stream and the deck is the deck stream issuing from a convergent nozzle. The design exit Mach numbers of the core and deck streams are M=1.6 and M=1 respectively. Results are obtained using the structured high-fidelity FDL3DI solver. A favorable comparison is obtained with available Schlieren and PIV visualizations. Even at design nozzle pressure and temperature ratios a shock train arises in the core stream due to the single-sided expansion but the deck stream is shock-free. Instantaneous and mean velocity profiles display the upward vectoring effect of the plume as observed in previous and ongoing experiments. A Kelvin-Helmholtz instability occurs in the shear layer formed by the core and deck streams. The resulting spanwise coherent structures breakdown in the presence of the deck boundary layer. The frequency associated with this instability is observed at downstream locations including through the shock train and greatly influences the flow near the deck in the centerline region of the configuration. The shocks are weaker near the sidewalls and the spectrum becomes broadband.