We present observations that reveal the existence of horizontally propagating, tidally-driven, high-frequency internal wave (IW) packets in a channel of the partially mixed St. Lawrence Estuary. The packets propagate transversely to the channel axis and collide with the shoaling lateral boundaries. The structure and energy of IWs are diagnosed with a two-dimensional, nonlinear nonhydrostatic model, and the results are compared with weakly nonlinear Korteweg-de-Vries (KdV) theory. The behavior of IWs running into the shoaling lateral boundary is examined in terms of published laboratory and numerical experiments. Our analysis indicates that IWs break on the slope, during which 6% of their energy is converted into potential energy through vertical mixing. The corresponding buoyancy flux, when averaged over the surf zone and the time of the mixing event, is more than an order of magnitude larger than values predicted by a published non-IWresolving three-dimensional (3D) baroclinic circulation model of the region. Even averaging across the full domain and tidal period yields mixing rates that are a significant proportion of those in the 3D circulation model. These indirect inferences suggest that wave-induced boundary mixing may be of general significance in partially mixed estuaries.