Laboratory experiments were conducted to determine the steady state internal wave response of a periodically forced long and narrow lake. The system is two-layer stratified and is subjected to periodic forcing over a wide frequency range. The ratio of the forcing frequency (f) to horizontal mode-one (H1) internal seiche frequency (fH1) governs the system response. For f ≥ 2fH1, highermode internal seiches were observed; for f ≤ (2/3)fH1, a non-resonant forced H1 internal seiche was observed; and for (2/3)fH1 < f < 2fH1, a resonant H1 internal seiche was observed. For the resonant regime, progressive nonlinear internal waves (e.g. solitary waves) formed upon the H1 seiche when the inverse Wedderburn number, W > 0.03 (W is the ratio of the wind-induced thermocline set-up to the thermocline depth). The nonlinear internal waves will break upon sloping topography at the lake perimeter, thus energizing the benthic boundary layer. Conversely, for W > 0.5, Kelvin-Helmholtz instabilities were observed to form within the progressive nonlinear internal waves, leading to diapycnal mixing (10 ms < Kρ < 10 −4 ms) within the basin interior.